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LT1 Stumbling / rough idle - 94 Trans Am

65K views 53 replies 8 participants last post by  1994 T/A Anniversary 
#1 ·
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 · (Edited by Moderator)
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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#3 · (Edited by Moderator)
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.
 
#4 ·
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.
 
#6 ·
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#9 ·
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/1/4th_gen_tech2.html#ect
 
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#11 ·
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.
 
#13 ·
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.
 
#15 ·
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.
 
#17 ·
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.
 
#20 ·
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.
 
#22 · (Edited by Moderator)
What makes you think the MSD is any different than "the same designed piece of crap thats in there now"?

There are some minor upgrades of components, and some added problems compared to a real GM/AC Delco/Delphi Optispark. In no way, shape or form is the MSD any better than the GM part, or worth the extra money. In fact, the parts (like the cap and rotor, for example) are fully interchangeable between the MSD and GM parts. MSD adds a "timing adjustment", that is of virtually no practical use, unless you have decided to install your camshaft advanced or retarded. The timing adjustment has no indicator on it, so you don't even know if it is adjusted correctly when you take it out of the box. The MSD rotor screws fall out as often, or maybe more often than the GM part. Why spend extra $$$ for a pretty red cap?

If you buy an MSD, the 93/94 version is "vented", and that is a plus, but its very easy to "vent" a GM 93/94 unit. There's a Corvette website with the instructions.

Get a GM Opti, from a discount GM parts dealer.

Or if you want something that is truly "different", research the "Dynaspark". But now you are over $600.
 
#23 · (Edited by Moderator)
I'm having the same symptoms in my '95 firebird/trans am. I'm interested which parts should and should not be replaced and what the optimum brands are for each. Optispark-Distributor-Rotor-??? I have to admit that I'm confused about exactly how many parts to do the job and take care of the arching/missing performance problem. After new plugs and wires and many other parts along with no more codes we are on to the Optispark problem as recommended by the dealer. Obiously, I don't want to go that route or I'll be buying the car all over again.
Not to hijack your thread, but it's very relevant and I think the parts question is valid for both of us.


Something like this intrigues:
http://www.autopartswarehouse.com/details/QQPontiacQQFirebirdQQAccelQQDistributorQQ19951997QQA3559125.html?apwcid=P1135867996W43b3f85c7ab9e&gan=1&apwidFXPCAIy, but necessary? Effective?

Better than the part from the page linked above? Here: http://shbox.com/1/Parts.htm . I just want to do it right...
 
#24 · (Edited by Moderator)
The Optispark is the distributor..... a single assembly. It consists of the body, which bolts to the front of the engine, and cap/rotor which bolts to the body. The body contains an optical cam position sensor. That provides the info the PCM needs to determine when to fire the fuel injectors and the spark plugs.

If you have a problem with the optical cam position sensor, it will set trouble codes. Those codes do not turn on the SES light, so you have to scan the PCM to see if you have codes for loss of the low resolution pulse signal and the high resolution pulse signal from the Opti.

The cap and rotor function exactly like any other distributor.... the rotor points to the correct button on the cap, and sends the spark out the connected wire to a specific spark plug. If you have problems with this section, there are no direct trouble codes. That is true of the entire hiogh voltage portion of the ignition - no codes for the cap & rotor, coil wire, coil, plug wires or plugs. In OBD-II (96 and newer) there are codes for the detection of misfires, so if you have an OBD-II PCM, you should be seeing codes for any serious misfires, and a scanner with "GM Enhanced Parameters" can actually recall the actual misfire count by individual cylinder. Those of us with OBD-I can not do this, because there is no misfire detection in OBD-I.

Appears the dealer has replaced everything else, and now is guessing its the Optispark. Has he scanned the PCM and found the codes for the Opti cam position sensor (DTC 16 and DTC 36 in OBD-I)? Has he put the ignition on an oscilliscope and evaluated the condition of each of the components (this step seems to be a lost art)?

With your 16-year old LT1, if the Opti has never been changed, AND there is some evidence that either the optical section or the cap & rotor is defective, I would replace the entire assembly. The 95 has a "vented" Opti, and that unit costs a bit less than the "unvented" unit used in 93/94.

In my opinon, the Accel Opti is not very good. There are only 3 units you want to look at - AC Delco, MSD and Dynaspark. All the others have extremely high failure rates.
 
#25 ·
Thanks, much appreciated!


Also know that my car has a 1996 Caprice engine in it (Police Interceptor, I've been told). Does this change the troubleshooting scenario? Love your car, btw. Mine is similarly green. I can say that we had all the codes reparied/cleared as of a few weeks ago, except the 26 code. We replaced EGR and no more codes. I'll post the more detailed repair history and code info/history after I survey this info and process it. I do have some recent info about the misfiring/arcing/cross-firing stuff and some from awhile ago when supposedly some parts were installed (and did not cure the issue completely). Thanks again.
 
#26 ·
OK, I am going to replace the Opti / waterpump / termostat / plug wires this in a few days. I found a Delphi unit for the replacement. I also bought the MSD rotor cap to make it vented. I took apart the opti and did a dry test fit to make sure all the pieces went together like they should. and they seem fine. I am going to seal it all up with some silicone and loctite the whole thing together tomorrow. Any words of wisdom before I do this? I have been reading some articles and it does not seem to be all that bad. I think a space issue is going to be the biggest problem. Any suggestions on replacing the plug wires...that looks worse than changing the optispark..LOL.

Wish me luck.....Im going in!!!!

MC
 
#27 · (Edited by Moderator)
Make sure you loc-tite the rotor screws. That is the critical part. My Opti was changed out for a vented unit when the stroker was built, and the engine was out of the car, so I've never had the pleasure of replacing one while the engine was still in the car. Make sure you have the air supply line and the vacuum line hooked up to the correct connections. The MSD cap must have both vent connections, which will make it different than the stock vented unit, because the stocker has the air supply connection on the body of the Opti, and only the vacuum connection on the cap:

http://shbox.com/1/opti_vacuum_harness.jpg
 
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