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LT1 Stumbling / rough idle - 94 Trans Am 1994 T/A Stumbling at low rpm / rough idle Rate Topic: -----

#1 User is offline   1994 T/A Anniversary 

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Posted 22 December 2010 - 09:39 AM

Ok, I have a 94 T/A that just turned 66K miles ( automatic ) and it recently developed a low RPM stumbling problem. When I 1st drive it, it seems fine. After a few min the stumbling starts. Car has no power at all and knocks, stumbles ( I don't hear any backfires ). I have to the press the gas peddle pretty hard so it can down shift and bring up the RPM's past 2000 - 2500. Once that happens, all the power seems to come back, the stumbling goes away and it seems to drive fine. Freeway driving for the most part is ok...going up hills on the side streets...not so much. While I am at a light, the car idles rough while in gear, if i put it in neutral it gets better, but not a whole lot. I noticed that my headlights flicker when the car idles rough ( ONLY WHEN IN GEAR ) and they do not flicker when in neutral. I had the codes pulled and I was told the car was not throwing any codes.. ( so i guess my car is fine and I am imagining all of this ). I was going to buy the 02 sensor and I have read that this problem that I am experiencing leans more toward that rather than the Optispark. I can pick up the 02 sensor ( BOSH ) for like $52. I keep seeing articles saying to replace both??? I see the one by the oil filter...where the hell is the other one? I have only had this car for a few weeks and its already driving me crazy. I REALLY want to keep it if I can get it fixed so I can enjoy it. Any ideas....anyone? Are BOSH O2 sensors good...seems like there is a HUGE debate between BOSH and AC DELCO.
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#2 User is offline   Injuneer 

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Posted 22 December 2010 - 09:52 AM

You have two O2 sensors. The one on the drivers side of the Y-pipe is in the base of the exhaust manifold. The one on the passenger side is partway down the passenger branch of the Y-pipe. They are both "pre-cat" sensors.

Do not use Bosch. Use OEM AC/Delco (now Delphi) parts only. You can get them discounted from online sources.

I'd do some investigative work first. The scan was a good first step. But there are several thing that can cause the engine to run poorly that are not picked up by codes in OBD-I. If the high voltage side of the ignition - coil, Opti cap/rotor, plug wires, plugs - is faulty, there are no codes. If the O2 sensors are still working but not putting out the correct, calibrated signal, its very possible there wil be no codes.

The fact that it appears to run OK after cold start, and then after a few minutes of warm, starts to run poorly, would indicate a possible problem with the O2 sensor(s). The PCM ignores the O2 sensors on startup, for the first 3 or 4 minutes, then when they are warmed up, and coolant is warmed up, the PCM switches to "closed loop" and uses the O2 sensor feedback to adjust the A/F ratio.

Any chance the coolant temp sensor is faulty? If that is telling the PCM the engine is cold, it will start fine cold, but then will run poorly when the engine heats up, and the PCM is still fueling it based on the engine being cold.

When you had the codes pulled, what tool did they use? Many people try and use a simple tool that shorts the ALDL pins to flash the codes on the SES light, and that won't work on a 94 or newer LT1.

Have the O2 sensors ever been replaced? .... plug wires? .... spark plugs?
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381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC engine management - 8 LS1 coils - 58mm TB - 78# injectors - 300-shot dry nitrous - TH400 - Gear Vendor O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

NO PHOTOS - MY WEBSITE IS BEING MOVED TO ANOTHER SERVER - SORRY FOR ANY INCONVENIENCE
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#3 User is offline   1994 T/A Anniversary 

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Posted 22 December 2010 - 10:03 AM

I have not checked the wires or plugs as of yet... I changed the oil, oil filter, coolant, rear end fluid, air filter..basically all the fluids since I had no idea when they were changed before I bought the car. As far as I know the 02 sensors have not been replaced> not sure exactly what tool was used to pull the codes,I was trying to watch him do it but really did not see much. he gave me a print out at the end but it really does not tell me anything. Not sure about the coolant sensor....however now that you mention that, the low coolant light comes on every once in a while for a short period of time then goes off. It can go days and days without coming on, then it will show up.

By the way.... the coolant light problem was an issue the 1st day i got the car...and it ran fine with the light on.

This post has been edited by 1994 T/A Anniversary: 22 December 2010 - 10:06 AM

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#4 User is offline   Injuneer 

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Posted 22 December 2010 - 11:17 AM

The low coolant light has nothing to do with the coolant temp sensor, or the PCM. The level switch is on the passenger side radiator end tank, a few inches below the fill cap. The switches are a very common problem. Always check to make sure its not low, and if its not low, the switch needs to be cleaned or replaced. Its only attached to the light on the dash, so it has no affect on the way things run.
1994 FIREBIRD FORMULA

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381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC engine management - 8 LS1 coils - 58mm TB - 78# injectors - 300-shot dry nitrous - TH400 - Gear Vendor O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

NO PHOTOS - MY WEBSITE IS BEING MOVED TO ANOTHER SERVER - SORRY FOR ANY INCONVENIENCE
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#5 User is offline   1994 T/A Anniversary 

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Posted 22 December 2010 - 11:47 AM

Thanks!! Ill look into that and check the plugs and wires next. If I do need to change out the 02 sensors, I found some pretty cheap ...$33 each..AC Delco AFS75... description says they will work with my car any thoughts?

I appreciate all your advise.

Mike

This post has been edited by 1994 T/A Anniversary: 22 December 2010 - 11:47 AM

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#6 User is offline   Injuneer 

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Posted 22 December 2010 - 11:58 AM

Correct part # for AC/Delco is AFS75, as you indicated.

You can check part #'s here:

http://shbox.com/1/Parts.htm
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FRED

Posted Image

381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC engine management - 8 LS1 coils - 58mm TB - 78# injectors - 300-shot dry nitrous - TH400 - Gear Vendor O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

NO PHOTOS - MY WEBSITE IS BEING MOVED TO ANOTHER SERVER - SORRY FOR ANY INCONVENIENCE
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#7 User is offline   1994 T/A Anniversary 

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Posted 22 December 2010 - 01:11 PM

That is great!! That list is going to come in handy... Thanks man! Have a great holiday.. Ill keep you posted as to what I do to the car and what works.

Mike
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#8 User is offline   1994 T/A Anniversary 

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Posted 27 December 2010 - 03:35 PM

OK, I replaced the TEMP SENSOR today ( AC Delco ) and the problem is still there. As soon as my O2 sensors arrive, I will change them out and see what happens.
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#9 User is offline   Injuneer 

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Posted 27 December 2010 - 06:06 PM

My suggestion would be that you test the parts before you randomly start replacing them..... gets very expensive to just replace parts.

Your temperature sensor could have been tested as described by Shoebox:

http://shbox.com/1/4..._tech2.html#ect
1994 FIREBIRD FORMULA

FRED

Posted Image

381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC engine management - 8 LS1 coils - 58mm TB - 78# injectors - 300-shot dry nitrous - TH400 - Gear Vendor O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

NO PHOTOS - MY WEBSITE IS BEING MOVED TO ANOTHER SERVER - SORRY FOR ANY INCONVENIENCE
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#10 User is offline   1994 T/A Anniversary 

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Posted 01 January 2011 - 03:13 PM

Changed both o2 sensors today... and...............nothing. Still doing the exact same thing. Car runs fine until it gets up to temp, then it stumbles at low rpm ( 1800 or so ) ....I am going to check the plugs/wires next... Any ideas after that?? Man I hope its not that damn Optispark. not a good way to start my new year.

This post has been edited by 1994 T/A Anniversary: 01 January 2011 - 03:14 PM

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#11 User is offline   philco 

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Posted 02 January 2011 - 02:04 PM

Check your coil wire. This wire runs from the optispark to the coil. If it is original, it is likely burned up and is grounding on the block causing your stumbling issues. If you even just moved the wire, it will likely change your symptoms intermittently at least. Basic car maintenance, the plug wires and the coil have to be changed periodically.
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#12 User is offline   WHITE-68 

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Posted 02 January 2011 - 02:11 PM

If it was a wire it would be immediate... Not wait till it warms up.. Unfortunately the optispark may have crapped on you..
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#13 User is offline   Injuneer 

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Posted 02 January 2011 - 02:39 PM

The IC module can be very sensitive to heat. The coil housing may loose dielectric strength as it warms up. A parts store should be able to test the IC module. But it may be necessary for them to heat it up to see if its a heat soak problem.
1994 FIREBIRD FORMULA

FRED

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381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC engine management - 8 LS1 coils - 58mm TB - 78# injectors - 300-shot dry nitrous - TH400 - Gear Vendor O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

NO PHOTOS - MY WEBSITE IS BEING MOVED TO ANOTHER SERVER - SORRY FOR ANY INCONVENIENCE
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#14 User is offline   1994 T/A Anniversary 

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Posted 03 January 2011 - 08:17 AM

OK....ill check these on these things and let you guys know what i find.....Thanks everyone.
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#15 User is offline   philco 

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Posted 03 January 2011 - 02:52 PM

In reference to white68's comment, is sometimes but not always necessarily true. The coil wire on LT1s is running from the distributor to the coil in such a manner as it may make contact with the waterpump or the block and if it is breaking down, can cause the stumbling you are having. Ive seen this symptom on an LT1 vette and an LT1 camaro 94 and 95 models. Sometimes even moving the wire can effect, reduce or even eliminate the symptom. Let's do some math: One set of plug wires 90 bucks and an hour. One Optispark distributor 300-400 dollars and one waterpump 180 dollars (do not consider reinstalling the old one if doing optispark job. Ok 580 bucks if you do it or over a grand if paying the mechanic. I've seen this and the effects aren't always immediate.


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#16 User is offline   1994 T/A Anniversary 

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Posted 03 January 2011 - 04:23 PM

Checked the coil wire and it looks almost new...no corrosion on the inside, wire itself looks perfectly fine. Plug wires look about the same as the coil wire from the outside anyway. I have not pulled the plugs out as of yet....thats next.
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#17 User is offline   -Greg- 

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Posted 03 January 2011 - 04:45 PM

hate to say it, but update this as soon as it's fixed i have the same problem that popped up today, i had to order a cable for that stupid obd1 comp. with obd2 adapter. but mines v6 it's got to be an ignition problem i just hope it's not going to bankrupt me.
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#18 User is offline   -Greg- 

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Posted 03 January 2011 - 06:37 PM

I just read about the mass air flow sensor that could cause that problem! i'll try it tomorrow and get back to you
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#19 User is offline   -Greg- 

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Posted 04 January 2011 - 05:17 PM

scratch that, i don't have a MAF, but i am trying a fuel system cleaner, good guy at autozone recommended it and hopefully in about 2 days i'll know, it's probably my fuel injectors or sparks, gotta be ignition or fuel system right?
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#20 User is offline   1994 T/A Anniversary 

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Posted 17 January 2011 - 12:04 PM

Just got my car back......Here is the verdict...... Water pump has a small leak.....that in turn messed up the Optispark. Was also told that there are a couple bad plug wires that are arching....... total damage... $2100.00. So.... I am sure as hell not gonna pay that and will do the work myself. I have been researching the Optisparks over the last couple weeks and came to the conclusion that the MSD seems to be the best one. I don't think sticking with the same designed piece of crap thats in there now is a smart move. Any thoughts on this? I see a lot of debate around this issue especially to make sure that the rotor cap is taken off and to apply some locktite on the screws then put it back together. Not looking forward to doing all the work myself, but I do want to drive the car again.
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