I own a 1991 corvette with the 350cu engine with 47,000 miles. The car will start and run fine for a few minutes then start running very rough. It will run so rough that it will shake the entire car badly. If I stop and let the car sit for a while then it will restart and run correctly before giving me problems again. Have taken it to a local Beaumont Chevy dealership to be looked at. The dealership has kept the car for 3 weeks and still hasn't found the problem. They say it's hard to diagnosis because it's "old technology" and most of the shop and service manuals for that year have been lost or destroyed. They agree there is a problem but they don't know what is causing it or how to fix it. Anyone that has any ideas about what could be causing the problem please let me know. I intend to take all your suggestions down to the dealership and have a long discussion.

 

Is the ignition coil pack wearing out. Sounds like it getting hot, and not producing the right voltage for the spark plugs to fire, causing it to run rough then shut down

 

Injectors, fuel pump and ignition module come to mind.
What have they tested ?
Do they know how to check for stored codes in ECM ?
Does the service engine soon light come on when it barely runs ?

 

It sounds like heat is a factory.

Did they actually check anything or simply let it sit there for 3 weeks?

 

The service engine light did not come on. I don't know what the dealership has checked ...... when I call the shop the service manager tells me they are looking at the car and trying to figure out what's wrong but because the car is so old they are having problems. I'm not a mechanic but I suggested a possible computer problem or maybe the exhaust pressure is building up ..... he said they would look into it. They are a new car dealership and I have had cars serviced there before so I guessed they know how to check codes.

 

The reality is they don't care other than to run the bill up drag the car to someone that actually cares and knows what they are doing. They are going to charge a fortune for I don't know as an answer. Send a truck and just get it out of there now. A non mechanic can change the ignition module and coil in an afternoon. A mechanic in a half hour. There are very few mechanics left the rest are parts changers. They plug into the ECM it tells them what to change and almost zero thought involved. The few talented mechanics left know why they need to change it and what it does but few and far between.
Dave

 

My first guess would be to remove the fuel cap when it is running rough and see if a clogged canister is the cause. If that is ok then check the slightly harder stuff.

You will want to know the fuel pressure when it is running ok and when it is rough.

You will want to see the spark using a spark test tool when it is running ok and rough and see if the spark dims.

Run the engine in the dark and check for spark leak.

Listen to each injector with a mech's stethoscope and see that they all sound the same will running ok and recheck when running rough.

Ohm out the injectors when it is cool and immediately after the rough running too.

Try disconnecting the O2 sensors to force the car to stay in open loop.

back probe the TPS to see that it swings smoothly from near zero to near +5VDC

Disconnect each vacuum line from the intake and plug it off. Start with the big one to the power booster.

Do the spark plugs look like each other? Do they match one of the diagnostic spark plug pics in the back of many cheapo repair guides?

What temp does the comp think the engine is running at?

What is ignition timing at idle when running ok? when running rough?

Basically you have a cheap to fix but PITA to diagnose problem. I would prefer that to an obvious expensive one.

The dealership you took it to should be ashamed.

 

If they can't fix or find the solution - why is it still there?

The first response is to hire a flatbed and tow it to a respectable shop which deals with that year's vintage. Many (not all) dealerships will end up milking your wallet dry for 'diagnostic time' without fixing your problem. Working with a Specialized shop not only knows your car better, but will work harder toward a solution and earning your trust for a future relationship with you and your Corvette.

 

Step one, do what ddahlgren said, get that car out of there NOW. Do you have a Corvette group in your area? If not there has to be an enthusiast who knows how to work on the system like the one in your Vette. A good backyard mechanic is a much better choice than the so called "mechanics" at some dealerships nowadays. I said "some" dealerships to be fair to the guys who are scarce nowadays who really know what they are doing. Having some dealership personnel work on your car is like getting change from Wal Mart, they can only give you what the machine says it right.
It sounds like you have either a heat related problem or something is happening when it goes to closed loop. Try unplugging the esc wire and start the car, if it stays running smooth it is likely the module inside the dist. and it can be tested.

 

- "old technology" - ??? - no, it's not old technology, its, so called, "mechanics" that are nothing more than parts changers, and without a FSM, have no idea what to do. the basic bosch fuel injection system used on the current LS-series corvettes is just an updated version of the old L98. do yourself a favor - tow that car over to a shop that employs mechanics that have real trouble-shooting skills!

 

Symptoms indicate injector or ignition control module problem.

Ethanol gas will cause GM injector failure.
Have someone ohm test the injectors cold then when the problem occurs, 14-16 ohms. Low ohm readings = injector failure.
Injector operating system is batch fire, one bad injector can cause problems for other cylinders or short the signal to all injectors = tow truck.
Many here myself included remove the GM injectors & install Bosh 3 injectors.

Check this link:
http://forums.corvetteforum.com/c4-t...questions.html

 

Hook up with the local Corvette crowd in your area ..... they should be able to recommend a good shop that can help you .....

 

This always makes me laugh. Any dealer that does any sort of auto repair should have access to online repair information through Alldata or something similar. We have it in our shop and I have found a complete duplicate of the FSM for my '87 there. In fact, it looks like every page of the FSM was scanned and then uploaded to the Alldata servers. Same pics, same text, and same page numbers.

I wonder what the dealer would do when cars with carburetors come in for service? Or early 70's cars and pickups with throttle body intakes?

I think the dealer simply doesn't want to do the work and you are probably being charged some pretty high labor rates compared to what independent shops would charge.

Is there a Corvette club in your area? If so, the members will most likely know of a shop or shops that do have the skills and tools to work on your car.

 

TPI can be very intimidating unless you roll up your sleeves and jump in. BUT you don't want them to learn on your car.
Get the thing home, and buy a FSM, and start reading. When you go through electrical, fuel, and emissions devices, you can instruct someone as to what you want checked if you don't actually want to fix it yourself....
But if it is in your garage, you can keep track of the time spent and the efforts performed.

Find anybody who has messed with any kind of fuel injection, and tell them what you want to check based on your FSM reading, and I bet your car will be running in no time.

 

Make sure you test fuel pressure...

 

Most of the dealerships don't have the FSM anymore . My friend who works at a stealership asked to borrow mine a while back.

 

You - "Hmm, you cant find my car's problem?"

Them - Nope, we ahh yadda yadda yadda b.s all the way.

You "Well that's funny I can't find my wallet either!"



As mentioned a few times already the BEST thing you can do at this point is to pay them a modest inspection fee and take your business elsewhere.

3 weeks! Pfffft!!! Guarantee your C4 is just sitting off in the corner and they only "try" something with it AFTER they fix all of the other cars and have NOTHING else to work on, even the ones that just pulled in a few days ago. You are essentially paying full rate for "spare time peanut gallery work".

 

Go get your car and RUN !

as everyone else has stated, that is INSULTING to have someone speak of "old technology" when its the same technology that is in use today, just a different package.

This is indicative of non-mechanics that are being paid to change parts, not fix or diagnose things. Its not that damn difficult. Hell, I'll call them and tell them what to do...IF they have the right tools! morons.....

start with fuel pressure at key ON and after resting for 2 hrs. Idle pressure as well. 40 psi key on, 38 idle and 20 after a 2 hr rest.

Injector ohms...no less t han 14 hot or cold.
ICM...$35 for a new ign module and ANY parts store can test...so a DEALER sure should be able to......

91...thats a speed density system IIRC...odd things those are. GM even thought so and quit using them after a yr or 2. The sensor on the plenum can fail or a vac leak can cause it lots of trouble.
Vac leaks...

Its a sure thing that the problem is covered in these lines above. A FSM has idiot-proof flow-charts that even a night school auto-tech grad could follow. Unfortunately, thats where most of the newer stealership techs come from. The same schools that offer classes in medical billing, copy machine repair, dental assisting and a doz other things that they rip off the state for by way of education grants and funding thru the unemployment system.

First step...get the car. 2nd, contact the local Corvette club or ANY hot-rod club and get some help there. These vette club folks are there to help and they can and will get you pointed in the right direction if they don;t fix it themselves..Corvette people are good people. Class acts.
Good luck!

 

Run Forest Run!!

There's enough information in this thread probably to find the problem.

But if you don’t want to work on it, put your efforts to find a good shop to fix the car. They are still around, you will probably have to do some networking with some other vette owners.

Where do you live?

 

, but disagree. I've seen the work of many a back yard guy. Its scary, down right hack jobs. Is that wiring held together with a house wire nut? I saw a rigged up house hold light switch for fog lamps once. that was interesting. No fuse either... The average guy does NOT have the knowledge of a good mechanic when it comes to cars.

I agree that many "mechanics" stink on ice. I've worked with a few. There are good ones around, I'd like to think I'm one of them.
Also, diagnosing and repairing today's vehicles is MUCH, MUCH harder than anything from the 80's back. They are WAY more complex. Try doing all this under a time constraint that is linked to your pay check. Change the way mechanics get paid and maybe you'll get better work quality. I pay financially because I'm honest and give a S***.

The general public thinks we just plug in the scan tool and magically fix the car, I wish it where that easy. To those folks my tool box is open for your use, I'll stand back and laugh at you. While you fail to fix it.

To OP: get that thing to a shop that has a clue.
This ain't rocket science.