The 1991 L98 engine problems continue......

[madcityman123]

I need help from all the sages on this sight as I continue to hit a brick wall! I took some advice posted and cleaned my t B on the motor to no avail. Now I turn the key, the car goes vroom then dies all in a nice and neat 3 seconds? Man how frustrating. I think my next move is to take the T B off the car and clean it good. What is the 3 prong electrical part on the passenger side of the TB? Where is the throttle screw located? Any other ideas please respond to thread.

Thanks in advance, newbie vette mechanic

 

[Hib Halverson]

All newbie vette mechanics, if they do not have the factory service manual, should stop what they are doing and buy one.

The throttle body does not have a "throttle screw" like on a carburated engine. The idle speed is controlled by the engine computer.

The telectrical part on the side of the throttle body is either the idle air control (IAC or "eye-ack") which controls the idle speed or the "throttle position sensor" or TPS which tells the engine computer how far open the throttle is.

If the engine fires and dies 2-3 seconds later, the first thing you might do is check the fuel pressure and check the fuel pump relay....well...that only after you buy a factory service manual.

 

[madcityman123]

Hib, don't you think if I had a factory service manual then I probably wouldn't have to come on here begging you guys for advice?

 

[KANE]

Hib, don't you think if I had a factory service manual then I probably wouldn't have to come on here begging you guys for advice?

...But you'd also know what you are doing!

 

[vetteboy86]

It would be a great investment to purchase the factory service manual. They aren't to salty, I actually bought mine off ebay!Has any work been done to the motor lately? First thing that comes to my mind would be a vacuum leak. My brothers 87 did this when the maf went south. You don't have one though, so possibly some other sensor.

 

[vette079]

Well, IF you really have the FSM, then you wouldn´t HAVE to ask what the little three prong thingy is, now would you.

IF you have the FSM, then you would KNOW where to start to troubleshoot.

The way you reacted to Hib´s reply indicates you have the FSM, right?

Now on topic: Might be numerous things that can be wrong:

Injectors, vacuum leak, fuel pressure (most likely), gaskets leaking.

Does it show codes?

EDIT: Ok, I saw your other post about the code, skip the last question.

Have you ohmed the injectors yet?

 

[cntrhub]

Do you need a manual for a carb and points car? Could you not walk up to any car in that era and know it was either, fuel, spark, compression?

Book me this... Compression is your only repair. The computer takes over your ignition and fuel. Could you now do the same is walk up to any car without a points gap or shake the car body, listen for gas, and if the car rolls down hill not jerking like pounding hard compression like you could take a finger and push the car is first gear on your ZF 6 speed is no compression at all?

We flip the car upside down without the safety deals in fuel injection, we could never turn the car off. So there are 3 key shut down the car deals is like the spark and pump are timed to drop a signal once you limit out the mile an hour on the rpm gauge.

There you have the car from floating valves is one deal. That is a loop closed and you find the fuel pump and spark drop are a few cylinders and lower fuel pressure is limiter action on the loop close to that red line on the tach; your engine is saved bye the telemetry.

So, are you witnessing a "shut down" event where it might be a crank sensor, a fuel pump, or a main sensor of 3 that more or less shut down the engine for good.

I would now try to eliminate the only 2 variables you can play with in the computer world. We know the engine lights up trying to start so eliminate, compression.

We have spark but for how long is now chase spark and fuel at the same time. Take WD-40 (never starting fluid is blow the heads off why don't cha), is if the WD spraying into the intake keeps the car running, then the crank sensor is good/ECM is good/Fuel pump feed to injector rail is in question.

I would assume there are no codes is because you mention none. I would assume the car is out of gas because the WD is now your gas tank. So, without going back and forth, crank and spray is see if the car keeps running is the spark shuts off or the fuel pressure comes on enough that there is now this...

Did you move anything on that throttle body where you touched the sensor out of phase of the body it sits in.

 

[TWISTERUP]

HUHHH??

 

[topless82vette]

Do you need a manual for a carb and points car? Could you not walk up to any car in that era and know it was either, fuel, spark, compression?

Book me this... Compression is your only repair. The computer takes over your ignition and fuel. Could you now do the same is walk up to any car without a points gap or shake the car body, listen for gas, and if the car rolls down hill not jerking like pounding hard compression like you could take a finger and push the car is first gear on your ZF 6 speed is no compression at all?

We flip the car upside down without the safety deals in fuel injection, we could never turn the car off. So there are 3 key shut down the car deals is like the spark and pump are timed to drop a signal once you limit out the mile an hour on the rpm gauge.

There you have the car from floating valves is one deal. That is a loop closed and you find the fuel pump and spark drop are a few cylinders and lower fuel pressure is limiter action on the loop close to that red line on the tach; your engine is saved bye the telemetry.

So, are you witnessing a "shut down" event where it might be a crank sensor, a fuel pump, or a main sensor of 3 that more or less shut down the engine for good.

I would now try to eliminate the only 2 variables you can play with in the computer world. We know the engine lights up trying to start so eliminate, compression.

We have spark but for how long is now chase spark and fuel at the same time. Take WD-40 (never starting fluid is blow the heads off why don't cha), is if the WD spraying into the intake keeps the car running, then the crank sensor is good/ECM is good/Fuel pump feed to injector rail is in question.

I would assume there are no codes is because you mention none. I would assume the car is out of gas because the WD is now your gas tank. So, without going back and forth, crank and spray is see if the car keeps running is the spark shuts off or the fuel pressure comes on enough that there is now this...

Did you move anything on that throttle body where you touched the sensor out of phase of the body it sits in.

I believe he said he has a 1991. I doubt it has a crank sensor at all, besides you can unplug almost all the sensors and the car will still run just not very good. Limp home mode!

Maybe you oil pressure enable switch is seeing pressure and thats why the car just immediately dies after a start, I kinda though if they dont see a certain amout of oil pressure after start it shuts the fuel pump off. Also doesn't the coil have two circuits one while it's cranking and and one while it's running? My friends 1990 had bad injector it ran just felt like you were pulling a boat with the car so I doubt thats your problem. Maybe a broken vacum line to the MAP sensor, egr valve is staying open. If it's a vacumn leak other then the MAP sensor it would have to be a large one to do what it is doing.

 

[diesel1456]

Every once in a blue moon mine will do that.. but i figure it's because i have some work done to it..I have that quick disconnect for the battery so i disconnect the battery and for some reason when the computer reset's itself it fire's right up..like i said this is rare but it has happend to me starts right up and dies..i never get any code's either..
just my 2 cents
Rich

 

[vette079]

You might not get any codes if the computer always resets after batt dis-and -connect.

 

[Hib Halverson]

Hib, don't you think if I had a factory service manual then I probably wouldn't have to come on here begging you guys for advice?

Ok. Here's the advice: purchase a factory service manual and diagnose the car's problems using it.

Once you know what the problem is, it will be much easier to fix it.

Guessing or trial and error waste of time and expense.

If you're going to "beg" for advice that advice will be most useful if you first diagnose the car properly, then post the symptoms based on that. Given the information you've posted to date, it's very difficult to offer any useful information.

 

[jmccloud]

Shuts off after 3 seconds

I once had a Buick Regal with your symptoms. One of my kids had given me an FSM and after several days of searching about in it, the advice was to check the ground to the coil pack (it was OK) and then the ECM. I don't know what your year Corvette calls the computer thingy that controls engine stuff but in my Buick it was an ECM. In my '95 Corvette, it's a PCM.
Whatever, if you've got an ECM and it's goofy, the service engine light won't come on and the radiator fan won't run when you use the paper clip method to retrieve codes (they should). At least that was the FSM's guide to my '91 Regals 3 second running symptom being a dead ECM. Or perhaps a '91 Corvette is totally different.
This is sort of far out but I can appreciate your need for help. If someone out there has a '91 FSM, chime in with what it says. Madcityman123 will buy the gospel FSM when he can!
Hope this helps.
PS I used to write equipment repair manuals for electronic industrial photographic equipment. I'd have been out of a job for writing such disorganized, poorly indexed, and poorly proof-read tomes as the FSM's. The FSM's are all there is other than us trying to help each other so please be kind and helpbul.

 

[LT4man]

Go to this link: http://helminc.com

Order a Factory Service Manual for your year Vette.

When it arrives, look up trouble shooting for driveability issues. The FSM will give you specific items to check. Follow the procedure step by step.

You will probably have to have a fuel pressure gauge.

The fact the computer is not recording any codes (only #12 which means electrical emission controls are A-OK!) is a very good thing.

As you collect data, report back to us and we will try to sort it out for you. There are many excellent technicians and regular folks who are willing to help you on this site.

BTW, if you start only one thread (not two as you have done) it will be easier for all of us to follow and try to help you through this diagnosis. Also, I believe it is in the CAC rules to only have one thread per problem.

SAVE THE

 

[madcityman123]

Go to this link: http://helminc.com

Order a Factory Service Manual for your year Vette.

When it arrives, look up trouble shooting for driveability issues. The FSM will give you specific items to check. Follow the procedure step by step.

You will probably have to have a fuel pressure gauge.

The fact the computer is not recording any codes (only #12 which means electrical emission controls are A-OK!) is a very good thing.

As you collect data, report back to us and we will try to sort it out for you. There are many excellent technicians and regular folks who are willing to help you on this site.

BTW, if you start only one thread (not two as you have done) it will be easier for all of us to follow and try to help you through this diagnosis. Also, I believe it is in the CAC rules to only have one thread per problem.

SAVE THE

Thanks for the direction. I however do not need any more lectures about getting the manual. I will get the manual but came on here for advice and help not lectures!! I DONT HAVE A MANUAL!!!!!!!!!! The arrogance of SOME this sight amazes me.

 

[madcityman123]

Go to this link: http://helminc.com

Order a Factory Service Manual for your year Vette.

When it arrives, look up trouble shooting for driveability issues. The FSM will give you specific items to check. Follow the procedure step by step.

You will probably have to have a fuel pressure gauge.

The fact the computer is not recording any codes (only #12 which means electrical emission controls are A-OK!) is a very good thing.

As you collect data, report back to us and we will try to sort it out for you. There are many excellent technicians and regular folks who are willing to help you on this site.

BTW, if you start only one thread (not two as you have done) it will be easier for all of us to follow and try to help you through this diagnosis. Also, I believe it is in the CAC rules to only have one thread per problem.

SAVE THE

Thanks for the direction. I however do not need any more lectures about getting the manual. I will get the manual but came on here for advice and help not lectures!! I DONT HAVE A MANUAL!!!!!!!!!! The arrogance of SOME on this sight amazes me.

 

[TWISTERUP]

Fuel Pressure Specs: 1) Ign. ON/not running > 40.5--47 psi

Test 2) engine running/idling-- pressure should be 3-10 psi LESS than first test.

 

[G Winter]

Thanks for the direction. I however do not need any more lectures about getting the manual. I will get the manual but came on here for advice and help not lectures!! I DONT HAVE A MANUAL!!!!!!!!!! The arrogance of SOME on this sight amazes me.

Seems someone picked the right name. With your attitude you might not get any more help. You just CAN"T trouble shoot these cars without the FSM !!!!

Glenn

 

[garyg]

starts up and dies

I have been experiencing this same problem on my 1988 C4 with 53,000 miles. It will start right up then die if I don't hold down the accelerator. I can keep it running but as soon as I lift my foot.. it dies. So far I had replaced the air filter and the fuel filter, I did get a code for the fuel pump relay so I replaced that. I just got a fuel pressure gauge tester and I'm getting correct fuel pressure.

I do not sense any vacuum leaks and the TB looks clean and the battery is charged.

I am not getting any codes other then 12. I am at a loss for a solution.

I would appreciate some advice.

P.S. I have a FSM and have read it over and over.

 

[TWISTERUP]

I have been experiencing this same problem on my 1988 C4 with 53,000 miles. It will start right up then die if I don't hold down the accelerator. I can keep it running but as soon as I lift my foot.. it dies. So far I had replaced the air filter and the fuel filter, I did get a code for the fuel pump relay so I replaced that. I just got a fuel pressure gauge tester and I'm getting correct fuel pressure.

I do not sense any vacuum leaks and the TB looks clean and the battery is charged.

I am not getting any codes other then 12. I am at a loss for a solution.

I would appreciate some advice.

P.S. I have a FSM and have read it over and over.

The first thing I would try would be unpluging the MAF sensor and see if it will start and idle if it idles plug the MAF connector back in--if the engine then dies the MAF is bad. 2nd I'd get some throttle cleaner with a spray nozzle and actually check for a vacuum leak ( I know you said you don't "sense" any) spray around the intake manifold/head area--the throttle body--vacuum hoses etc. also check the bellows hose between the MAF and the throttle body for splits/cracks--finally if you just barely have to hold your foot on the gas to keep it running it could be the IAC valve. In your FSM there should be a area for diagnosing your symptoms WITHOUT any codes--there should be a list of things to check for your particular problem,do not miss any steps--if you don't understand the "test/checks" post back someone will likely be able to help. It's a good idea to start your own thread with your own symptoms/problems-more members will likely see your post then.