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1993 6-spd T/A - 1996 C4
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Discussion Starter · #1 ·
Gen 2 LT1 4l60e. Experienceing extended "cold" starts and what I beleive is low fuel economy (13.2MPG city on E0 driving easy). Engine seems to be running normally once started. I put a pressure guage on it today...

First, heard a quick psst when I cracked the cap of the Schrader Valve. Did not see any fuel escape when cap was removed.

Now...

Key on / prime. Pressure shoots up instantly to mid-40s and just as quickly falls to zero.

Key / start. Fuel pressure shoots up near 50 and engine starts. Pressure immediately falls to around 36 and holds steady. Quick blips of the throttle sees the pressure up to 40 or so and right back down. I beleive this to be normal operation.

Key / engine off. Fuel pressure instantly falls to zero.

Engine could be slow to revv, my '93 definitely goes up-and-down before this one gets up and feels several times more responsive in this respect. ...but the '93 might not be stock. I don't know how quick an "idle to redline to idle" cycle should be on a stock LT1.

No issues with idle or acceleration. No stumbling, stalling, or running rough. No fuel scent from oil. Just extended starts and what I consider to be poor fuel economy. I'm under the impression the LT1/4l60e combination is good for 18MPG city on E0. My other LT1 gets 13MPG on E10 and 18 on E0 under very spirited driving. Driving it "easy" like this, I could pull upwards of 24, so 13 seems very low to me.

When I ran a live-data scan a couple months ago to look into this, everything appeared within specs. I think long term fuel trims were 128 and short was 0 on both banks. Only DTCs were for a burned out SES bulb and the QDM driver or something related to the fans.


Assuming FPR and I can stop diagnosing here? ...even though the engine seems fine?

...and, again, assuming the FPR is at fault, is Delphi still worth it? Delphi is more expensive and has a much shlrter warranty than everything else. I know Delphi was good at one time, but times have certainly changed.

Thanks!
 

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With the fuel pressure falling so fast after a key on prime, have you considered the check ball in the fuel pump as the problem.

My take is that once engine starts, fuel pump can run as long as needed to keep fuel pressure up.

I rule out a leaking fuel injector because that should cause running problems due to incorrect fuel/air mixture.

It could be pressure regulator. To rule out that problem, have your stethoscope on return fuel line. Have assistant prime fuel rail. listen for sound of fuel returning to tank.

The stethoscope should also work for testing a bad check ball in pump. Have it on fuel input line. Again have assistant prime fuel rail. Listen for fuel returning to tank via input fuel line.
 

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I spent too much time typing this….. had to finish my dessert :). Appears sea dog covered most of this already.

Did you go back later and check the Schrader valve for leakage? It doesn’t take much in the way of fuel loss to drop pressure to “0”. Tighten or replace the core just to be sure. The extended start and rapid pressure loss sure sounds like something is leaking. Injector, fuel pressure regulator shutoff, fuel pressure regulator diaphragm, pump check valve, fuel line, including any flex line in the tank.

The leak may be small enough that it holds pressure spec at idle, or even revving the engine under no load. If you looked at the LTFT for the idle cell (16) it could be OK even with a leak. What scanner are you using? The OBD-2 units usually report the fuel trims in +/-%. In that format, 128=0%, numbers above 128 are +%, below are -%.

Or the issue might be something different in the Corvette fuel system that doesn't hold the pressure. I'll check my Corvette fuel injection reference manual.

The quad drive code is unusual too. Your 93 ECM was the last one on an F-Body that used quad drivers. I have the Corvette code lists, what was the code? I can check that too.

I think Delphi was totally divorced by GM, except for some steering components, which GM absorbed. Delphi went bankrupt and eventually closed almost all of their US plants. I would use AC Delco.
 
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1993 6-spd T/A - 1996 C4
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Discussion Starter · #4 ·
OK... Probably the check valve in the pump assembly as SeaDog suggested. ...and leave it to me to throw out the G** D***** folder of receipts the Previous Owner handed me, so I'm out a few hundred dollars if I wish to pursue it. Not going to if I'm being honest.

I managed to get some time under the hood today. I cut the wire P.O. ran directly to the PCM from chassis ground to "hotwire" the cooling fans and placed a 35A switch in-line just in case the fans don't come on after cutting the wire. Before starting the engine, I cleared all codes. Cooling fans come on with the A/C, and go off about 2 minutes after A/C is switched off. If left to it's own devices, fans come on at 228/230F and appear to turn off around 216/218F going by the digital gauge. My '93 T/A runs much cooler in 10F higher ambient temps, sitting still at around 195/200F, but I think the extra temp is down to the engine compartment differences. Or...

Anyway, after a few fan cycles, I pulled codes. Now, just the one.

P1642: QDM/ODM "A" - Output 2

I suspect this is related to a fan relay. I have to find out the actual OEM fan programming and compare temperatures. Maybe the fans are not coming on "low speed" when commanded but do come on high speed? I don't know. I'm familiar with the "Fan 1, Fan 2" setup, not the "high/low speed" setup. Apples and Oranges apparently. I can say that when I flip the switch to enable the fans while they are ON, fan RPM does not change. Speed sounds the same as when the A/C is on. ...assuming that's "high"?

Of course, maybe I have to run through a few ignition cycles to clear the code, but I've been clearing the code in the scanner application, so I guess we shall see. I just wish there was consensus on what LT1 fan temps should be. I have yet to see the same numbers twice online. Everyone says something different. Knowing the actual low and high speed temps would go a long way for me. The more I think about it, the more I'm thinking I'm not getting low speed operation.

Did a sensor scan today, everything seems normal. I wish the scanner application I'm using showed information like Scan9495 does. It would be very helpful to see the low/high speed "command" from the PCM visually. Maybe the app does display this and I haven't found it yet. Every parameter OBD2 has from '96 through today is in my application, so there is a lot to scroll through, and I'm not always patient. Knowing I can just run 2 jumper wires from my OBD1 ALDL connector to the OBD2 port, maybe I'll download "GDScan" to my tablet and see how close Gary came to Scan9495 with the interface and features. ...always wondered what "GDScan" was. GaryDougScan! ...for OBD2! I always thought it was one of the "free" apps Gary used as an example for Scan9495, an alternative to Scan9495, or an "alpha version" of Scan9495. NOPE! Guess he wrote GDScan so he could have a scanner for his '98 T/A. Even though he owns a Tech-II.

At IDLE, 225F, Closed Loop after a 5 mile drive:

Bank 1 Short Term Fuel Trim: -0.8%
Bank 1 Long Term Fuel Trim: -3.9%
Bank 2 Short Term Fuel Trim: -1.6%
Bank 2 Long Term Fuel Trim: -1.6%

MAF: 0.9lb/min
MAP: 10.3 inHg
IAT: 136F

Absolute Throttle Position: 0%
Engine Load: 2.4%
RPM in Park: 691 (675-709)

Looking over the original codes I had upon purchase, in addition to the P1642, I also had two "unknown" codes, P0719, P1441, P1641, and P1661. Seems like I have resolved all but the P1642 at this point.

...getting there!

So, what does the P1642 point at? One of the reasons I went with the scan tool I have for OBD2 was for it's detailed "Repair Reports", well, for this particular error there is no "report" available unless I write and submit it.

OF COURSE!!!




Edit: Didn't think about looking into the "freeze frame" data, never had that available before. I'll update with what was there when I scan again tomorrow evening. I'll have a few ignition cycles by then.
 

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Is it possible there were two jumper wires installed to make the low/high fans work? Was there a switch somewhere in the circuit? This is the correct way to wire a switch with jumper wires:


Are you aware the 1993 dash coolant temp gauge appears to be mis-marked. The lowest line is marked “100”. On all other years it is marked “160”. That led to a very popular misconception that the 93’s run much cooler than other years.


P1642 is for “Fan Relays #2 and #3 control circuit”. That's based on an F-Body DTC list. The Corvette code list in my Corvette fuel injection manual shows P1641 as “Fuel Relay #1….. “, and P1642 as “Fuel Relays #2 and #3”…..” but that appears to be a typo. Unfortunately I have found several errors in the manual, including one section where they reverse the terms “open loop” and “closed loop”. The book was not written by GM, but bears a GM part number. A lot of the info in it could only have come from GM. A quick look at the 1996 Corvette diagrams shows only one fuel relay.

Relays #2 and #3 are the relays that switch the fan power circuit from the two fans in series (low speed) to two fans in parallel (high speed). That would explain why you aren’t seeing the fans speed up. A/C on turns them on at low speed.

You have probably seen the diagram for the 3-relay high/low setup, but if not, it's the bottom 1/2 of this:


The Corvette 96 diagram is the same. Even the wiring colors are the same. Use the 1996 factory service manual troubleshooting chart for P1642.
 
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1993 6-spd T/A - 1996 C4
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Discussion Starter · #6 ·
Is it possible there were two jumper wires installed to make the low/high fans work? Was there a switch somewhere in the circuit? This is the correct way to wire a switch with jumper wires:

It is possible there are two wires, but I do not think so. I'll have to look deeper. The fan relays appear to be on the radiator support and there are no modifications to their harness. That I know, it was the first place I looked for the bypass. The only thing I have seen out of place so far is a 16GA speaker wire (Monster Cable) going from the frame and into the PCM harness right where the plastic loom stops short of the connectors. I assumed he just grounded the wire that the PCM normally switches the ground of internally to power the relays. Until I saw the wiring schematic you linked for a proper bypass, I thought what he did was correct, I was just upset where he did it. I'd have done it at the fan harness, not inches from the PCM connector. I'll look again though, and I'll split the loom to see the actual splice, but I don't think I'll find anything more than a hack job to a single wire with a single wire.

There aren't any switches I am aware of. Nothing under-hood for sure, nothing obvious in the cabin and I've had both knee pads down. To my knowledge, I added the only switch. When I cut that 16GA wire, the fans did not come on with the key any more. I wired up the switch, flipped it, they came on.

I left it "Off", started the engine, and when I turned on the A/C, the fans came on. I shut the A/C off and a minute or two later the fans went off. When the engine hit a temperature they came on, and eventually went off again after a couple minutes. The system seems to be acting normally. Thank the gods. No PCM damage which I was afraid of. ...even more so now that I see the right way to do this involved Diodes.

Again, I'm almost 100% positive this bypass was nothing more than a single jumper wire to the chassis. I'll have to verify fan speed operation. It appears I might have not hit the high-speed temperature threshold today. I think it's 238F (seems high) from the most recent Google search, and the highest I saw today was 235F, so maybe high-speed works. That's the next test I guess. BTW, I'm getting these temps from the digital Information Center, not the "analog" gauge.

I do not plan to use the switch. Not even going to mount it. If all goes well, I'll remove the bypass-wire completely and repair the factory wire where it's spliced. The switch was installed because thinking it through, I honestly feared the PCM was damaged and would not be able to command the fans. I thought a properly sized switch was better than a blob of solder and some electrical tape if things didn't pan out.

Are you aware the 1993 dash coolant temp gauge appears to be mis-marked. The lowest line is marked “100”. On all other years it is marked “160”. That led to a very popular misconception that the 93’s run much cooler than other years.

That I was aware of, thanks to you sometime back in 2006. Once Gary gave me Scan9495, I started going by the ECM temp. It's been a while since I sat and watched temps on the '93, but once I got the cooling system completely sorted, I don't think it ever went over 220F even when idling endlessly on the hottest highest humidity days. When I picked up this C4, the next day when I returned the plate, it began overheating in his driveway after about 5 minutes, and a week after that, I was in a lot of stop-and-go traffic and it began to overheat. As soon as the car is in motion, temps fall to normal and remain there. The engine does get sluggish in slow/stopped traffic, so I know it is overheating slightly. Right now though, I can control my driving situation so I just don't go out in heavy traffic, but at the same time, I'm not afraid to.

My '93 did this for years and I eventually traced it to a burned out "secondary" fan motor thanks to Scan9495. I was absolutely amazed at how well my temperature was regulated from then on out with two functioning motors. Go figure. ...of course the C4 could just run hotter, but I don't think it should get sluggish in traffic. I'll get out the measuring tape out next time the hood is up on the '93, but looking at the fans on the '96, they look small. I mean really small, so the '96 could be somewhat deficient in cooling capacity as-is in comparison. I'll start with learning the exact fan thresholds, and seeing if they're doing what they should when they should. I know at the very least I have low speed, and the PCM can command it. Getting the system sorted out though means verifying my second fan speed. Hopefully, it's as simple as a new relay or two. Maybe half the realys are burned out? I dunno. Thinking out loud here.

My hurdle is the lack of definitive fan temp thresholds AND the fact the Y-body guys are split into two camps. 50% say this is normal behavior for a C4 and suggest all kinds of mods to resolve it, and 50% agree with us, a properly maintained LT1 cooling system is more than capable and "slightly overheating" under any circumstance is NOT NORMAL. Those itty-bitty cooling fans though, each is roughly the size of the fan on my '85 Goldwing. I'd say they look too small for automotive application and aren't correct, but they fit and fill the space...


P1642 is for “Fan Relays #2 and #3 control circuit”. That's based on an F-Body DTC list. The Corvette code list in my Corvette fuel injection manual shows P1641 as “Fuel Relay #1….. “, and P1642 as “Fuel Relays #2 and #3”…..” but that appears to be a typo. Unfortunately I have found several errors in the manual, including one section where they reverse the terms “open loop” and “closed loop”. The book was not written by GM, but bears a GM part number. A lot of the info in it could only have come from GM. A quick look at the 1996 Corvette diagrams shows only one fuel relay.

Relays #2 and #3 are the relays that switch the fan power circuit from the two fans in series (low speed) to two fans in parallel (high speed). That would explain why you aren’t seeing the fans speed up. A/C on turns them on at low speed.

You have probably seen the diagram for the 3-relay high/low setup, but if not, it's the bottom 1/2 of this:

Crossing my fingers it's the #2 and #3 relay. After I verify I do NOT have high-speed, I'll go straight to them. I haven't had A/C in the '93 since 2005, and I have a different fan setup, but I always thought both fans came on with A/C in the '93, so I assumed that would be high-speed on the '96. If A/C activates low-speed, I might have shut down 3 degrees before the high-speed threshold today, or, I have no high-speed fans. I'm betting on no high-speed based on what I said above about how it acts in heavy stop-and-go traffic or sitting still a while. I bet I'm missing that extra bit of cooling when I really need it.

It'll be just my luck that the PCM is actually damaged due to the lack of diodes in the bypass "non-circuit" used here and the P1642 is a result of failure to command the high-speed switch-over. Praying it's just the relays. It makes sense the more I think about it. ...but I'm expecting the PCM. Those diodes are in the "proper" bypass circuit diagram for a reason.

I do need to get the FSM, but too much money has been flying out. Between the house, kid, ex-wife, 2 cats, electronics, appliances, 6 days driving the Blue Ridge Mountains, and 3 other cars, the C4 has taken the back seat as far as projects are concerned. I swear to God that if I kept my savings, I wouldn't have unusual expenses. ...but no. I gave up my $10k safety-net and Karma said "Hey Tony! ...". I did drop $400 in subwoofers I wasn't planning on in July, but it was my birthday. I went out to buy a pre-built box for the same cost as materials, and well, I got a little out of hand. Happy Birthday to Me!

Now we are heading into Halloween, Thanksgiving, two Birthdays, Property Taxes, and Christmas. I'm literally spent until next February. Seriously. ...but this is every year for me. Patience!

A couple fan relays? I can swing that. Best case scenario at this point. I'm just so afraid that both speeds are functioning because I can't find anything else wrong to explain the minor overheating in stop-and-go traffic.

FSM is planned though. I want the paperbacks, not the pirated CDROM. Inaccurate it may be, but I was so unaware of all the "systems" in this thing that can throw codes of their own. CRYPTIC codes. It's insane, for what little bit of car is actually here, it appears there's a figurative 7 Series BMW's worth of "non-standard / proprietary" codes.

The Corvette 96 diagram is the same. Even the wiring colors are the same. Use the 1996 factory service manual troubleshooting chart for P1642.
I was hoping that was the case. Thank you for taking the time to check on that. I have been glancing over the F-body manual wondering how close they are. I can live without the wiring colors matching, but having the proper DTC flow-charts and measurements is a BIG deal. I use the DTC section most anyway. Sometime next year though I want to go over all the ground points and I'm going to need the Y-body FSM for that. Found one ground point when installing my stereo and it was nice and green. I can only assume all the others (and I hear there are many) are too.
 

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P1642 can apply to either the relays or the wiring. Previous owner may have had his way with it. And the diagnostic is only checking the integrity of the signal side of the circuit (PCM to relay), not the load side (battery to relay to fans).

The F-Body temps are definitely 226°F (low) / 235°F (high). Looking around the Corvette forums, they seemed somewhat consistent with the 226, but all over the place on the high speed temp.
 

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1993 6-spd T/A - 1996 C4
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Discussion Starter · #8 ·
Well, it appears the Freeze-Frame data isn't going to tell me anything. Apparently, only the PIDs I graph are reported in the freeze-frame data, which is dumb. Looking at the data, everything appears normal. The only thing that may be of relevance is engine temperature. ...but it's at the low-speed threshold and the fans come on low-speed for sure. Unfortunately, the weather hasn't been cooperating. It's either been "not hot enough", or hot enough but raining too hard to go out. Weather-stripping is in too bad of shape to go out in anything more than a sprinkle. I'll graph all available PIDs and try again, maybe we'll get more useful information. Tomorrow might be good for high temps and no rain, if not, Friday or the weekend should be miserable enough. For now though, the data is basically what I already posted, so pointless.

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Note that the code has two different meanings:

1 - the fans were supposed to be on (relays closed) high speed, but the voltage on the signal side of relays 2 and 3 indicated an open circuit, or

2 - the fans were NOT supposed to be on (relays open) high speed, but the voltage on the signal side of relays 2 and 3 indicated a closed circuit.

This could be the result of the previous owner adding or deleting a wire while he altering the system incorrectly with the single wire you found. As complicated as that 3-relay circuit is, i needs to be traced and verified. Who knows where the previous owner added or cut a wire. As you noted earlier, the fans only run on one speed.

It is possible when the coolant hit 107°C (224.6°F) that only the fan low speed was supposed to turn on. But the diagnostic saw the fans come on at high speeds, indicating the signal side of relays 2 and 3 were at the wrong voltage. The commonly quoted figure is 226°F, but this could be an issue of whether the PCM works on Fahrenheit or Celsius.

The diagnostic requires certain conditions on other sensors to be met before it can be run, so that may be the reason for the data selected to be shown. I honestly don't know, just speculation. Actually, everything looks pretty good.
 
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84 T/A 301T T5, 3.73s
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Whoa, this thread is pretty deep. When I have time, I'll need-ta check it out. Maybe I can get something out of it, cause my JD 214 with a 14HP kohler eng., (that I put an electric F/P on), ran for 45 mins., then starting spitting fuel out the venturi. Was told I need a regulator. Not tryin to hi-jack this thread, just talkin out loud. :whistle:
 

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1993 6-spd T/A - 1996 C4
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Discussion Starter · #11 ·
Well, I was wrong. It's not a single wire butchering another single wire. The --- ------ Son of a ----- cut BOTH fan wires 1/2" from the PCM and connected both together with a barrel connector, adding the ground in one end. I want to drive to his house and punch him in the face now. The connection is so close to the PCM, by the time I cut and strip the wires, I'll be against the PCM. Looks like he cut the "slack" created by the barrel connector to, so maybe not even enough harness wire to repair. I probably have to add a jumper wire in-between, so likely 2 barrel connectors on each wire. At this point, my gears are grinding. GRRRRR!!!!

Pretty sure now this problem will be resolved as soon as I separate the Blue and Green wires from each other. PCM is clearly seeing voltage on the high-speed circuit every time it grounds the low-speed circuit. If both fans are indeed running "high" all the time, then this doesn't explain why I'm overheating in rush hour traffic. I guess trying to get the engine hot enough to see if the high-speed relay takes over will have to wait until I get these wires sorted out. Getting there though I guess.

Won't be able to get around to this until the weekend. Maybe I'll go yank the rear drivers side wheel, remove the fender liner, and disconnect the power antenna instead. I never listen to "Over the Air" broadcasts, so there is no point in listening to the antenna motor grind every time I turn the key on or off, or for the antenna to go up at all. That'll at least be one project checked-off this week.

...just, how dumb is this? The guy couldn't do this splice father away from the PCM? What goes through these peoples minds?

Electrical wiring Cable Gas Wire Electronic device
 

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Yep…. hidden sh_t. It’s hard to believe people can get so sloppy with their amateur attempts at modifications. Glad you were able to track down this screwup.
 

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Discussion Starter · #13 ·
Yep…. hidden sh_t. It’s hard to believe people can get so sloppy with their amateur attempts at modifications. Glad you were able to track down this screwup.
The way he had this all taped off, it looked like the splice was done where the harness forks off about 12" from the PCM. ...NOPE!

Thanks for everything so far. I truly appreciate it.

Still working on the original problem though, at least 5MPG is missing with no indication of where it went (using E0). FPR and fuel pump are fine, fuel trims appear pretty good, no performance issues. Literally idling down the road in OD at 35 MPH, and getting 13MPG? Something isn't right somewhere. Gear ratios say this should get very similar MPG than my T/A. 4l60e has a 0.696 OD and a 3.07 rear, my T/A has a 0.62 OD and a 3.23 rear. There is only, what? 0.13 difference when multiplied out?

Could a 0.13 difference in ratio equate to 5MPG just cruising around? ~5% difference in ratio causing a >20% loss in fuel economy? Doesn't seem right, but I'm running out of things to point my finger at. If it's not electrical or fuel related, it's got to be mechanical. Maybe torque converter slipping? ...but I assume there's a code for that too.

Very lost here.
 
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