Mike:

I just bought a 1991 Corvette and it has a terrible surging problem. I believe it is vacuum related because with throttle above 1000 RPM the car does not seem to surge. The surge is from about 400(450) to 800(850). The engine feels like it wants to idle at 1000 RPM and that would be fine with me. In drive, foot on the brake the car's surging causes the engine to shake.

I have already changed the spark plug wires and will attempt to change the spark plugs tomorrow.

I can not find a complete vacuum diagram for a 1991 Automatic L98. I did notice a lack of a charcoal canister and 2 bare(unused) barbs on the throttle body.



Lower barb goes to heater core?
Middle barb has a very strong vaccum
Upper barb does not have vaccum or pressure


Passenger valve cover was hooked up to middle barb on TB


Driver Side Intake Manifold


Passenger Side Intake Manifold


Driver Side Radiator Hose. Bulging is evidence of overheating, yes?


Vacuum intersection at the firewall on the driver side?



That intersection leads to this, further forward on the driver side.


Brake Booster?


My diagram says there should be a canister on the forward driver side just behind the bumper?




< Check gauges Light is ON
< Change oil light is ON
< Car idle rapidly roams constantly below 1000 RPM from 450-850 RPM
< Engine will surge in drive, vehicle moving and will pull itself forward, slight jerking movement
< Presumed to be stock/original plugs, cap, rotor

Cliff Harris

The TOP barb in the first picture should go to the passenger side valve cover. It feeds filtered air into the crankcase, which gets sucked out by the vacuum on the PCV valve in the driver's side valve cover.

Try plugging the middle barb (for now). My car doesn't have one of those so I can't help you on its purpose (but see below).

The missing runner to plenum bolts are probably contributing to a vacuum leak.

The vacuum T in the 7th picture goes to the cruise control and to the vacuum tank (the round thing in the last picture.

The charcoal canister is missing. It gets vacuum from the throttle body. On my car the vacuum source is on the bottom of the throttle body in the middle but that could be the center barb on the passenger side of your car.

Take a picture of the PCV valve and hose in the driver's side valve cover. On my car it connects to a barb on the intake manifold between the runners (in the center on the driver's side).

Mike:

I had the top barb to passenger valve cover with the middle barb plugged and then switched the middle and top barb. Same surge, if the top barb to VC is the correct route then thats what I will do.

Attached is the PCV valve to IM



This is on passenger VC and the line runs to the passenger fender and connects to another line. I imagine this sensor get input from the line and outputs a digital signal



"The missing runner to plenum bolts are probably contributing to a vacuum leak."

The holes are threaded but it doesn't look like the hole goes through the intake manifold. I assumed it was for mounting something to the intake manifold.

Mike:

Took the vette for its first real drive today, towed it home. At the parts store(where it broke down) convieniently was a corvette shop and I was told that the distributor internals heat up and send no spark/low spark to the sparkplugs which is why it won't start and when it did it ran awful. I will be taking the distributor apart and will be diagnosing the issue, just wanted to hear if this actually a "common part, common problem" and any reccomendations

Mike:

I have replaced the distributor cap, rotor, ignition module, spark plugs and spark plug wires. The car starts up great, full throttle response in neutral (ignition module fixed the lack of response), but in drive the idle slightly surges, the car shakes and wants to push forward. After no more than 2 minutes in drive the car idle drops to 400 and then promptly dies. The car will start and die twice but will not start after that. I just purchased the only pick-up coil in south florida and I will ohm test the old coil tomorrow but expect it to fail.


While replacing the ignition module I noticed that the black plastic that separates the wires had disintegrated and fallen into the screen at the top of the distributor. I was careful to not further break the brittle connectors but didn't feel confident that the plastic would last so I used marine grade heat shrink with the adhesive lining and heat shrinked the wires onto the ignition module.

I also noticed the TPS sensor connecting clips have broken off and after pulling the plug out noticed the plug is shot. Not much experience with vettes but logic would suggest that if the computer does not know where the throttle is at, it can not send the right amount of fuel so it dies?


I've had the car less than a week and would like to actually drive it so please chime in and help me out

Mike:

Brittle plastic, I ended up with about a handful of broken miniature plastic pieces. Its probably not that clear and I don't seem to have a picture but most of the old plastic was in small pieces on top of the screen in the distributor


Marine grade heat shrink with adhesive. Yes I know the yellow is just regular heat shrink, I didn't have marine grade in that size

Mike:

Any helpful hints from anyone that has similar issues would be greatly appreciated

c4cruiser

Vacuum leaks can be a royal PITA to find. One thing you really need to do if you are going to work on the car is buying the Factory Service Manual or "FSM" for your car year. It's the same book set that GM techs used for repair procedures.

Is there a decal on the radiator shroud that shows the vacuum line routing? If so, follow the lines and start by plugging every open line or port to see if that changes the idle surge.

The TPS wire plug should be available. It might be a good idea to replace the TPS and remove the throttle body to clean everything up. It's not uncommon for the TB butterfly shaft to cause the holes in the TB to elongate; that can introduce extra air into the intake.

Not having the Evap canister or the relay that sits next to it is most likely the reason for the SES light being on. Pull the codes and post them here. That will go a long way in helping with the diagnosis.

Mike:

Thanks I have a neighbor that has a code puller so I'll get to that today. The Check Gauges Light comes on after about a minute of being on, I had the LCD cluster display changed and its output is a series of vertical and horizontal lines, I believe it is from a different C4 and is not the exact one for a 91 so that could be my issue for the CG light. I read that the light can/will come on if the fuel is low and if the cluster doesn't know how much fuel is in the tank its just turns on. The tank has a full tank of premium gas, which equals about a month of gas for my previous vehicle, 2.0l civic.

My only gripe with it being a vaccum leak is that the original owner recently, days before selling me the car went on a 150+ mile trip and now the car runs like crap.

I'll pull the the throttle body, clean in, try to find a new tps, get a plug and go from there. But first I want to ohm test my ignition pick-up coil before I replace it.



Do any other GM vehicles, same era have the same canister that I can get used. I don't want to spend $100+ at a local corvette parts shop

c4cruiser

An empty gas tank will not cause a CEL. If nothing else you can easily burn up the fuel pump by running the tank empty.

The vapor canisters are different through the various years. RockAuto has them for your car for about $60 AC-Delco #21575 or Standard CP-1031 Looks like you will need the Purge Valve and the Solenoid and some vacuum hose.

There should be a pair of steel lines that run from near the upper A-arm down along the frame rail to just in front of the top part of the forward wheel well I don't see them in the last pic (where the canister would be). Not sure if those red wires with the crimp connectors are for the solenoid, but it does need wiring to it. Could be the prior owner had something else there.

Pic of the LCD display cluster?? For a '91, it should be this one:

Mike:

I have at least a 90% full tank of premium.

Long story short for the cluster, I brought it in for a nonworking tach and a dimmly lit lcd panel. They repaired and replaced everything needed for a working tachometer, tachometer is responsive and fully working. However when they changed the lcd display they must have used one for a different model c4. I have pictures in my other thread.


So today I pulled the distributor and changed the pick-up coil, reinstalled and the car was somewhat better, full throttle response in park, put it into drive, went around the block and everything was great but soon the idle dropped and the car barely made it back into the driveway. Something(sensor?) heats up and does not want to play along, idle drops car dies and then will not start. All that is left of the stock ignition is the coil that sits on the top of the distributor.


I have never had the CHECK ENGINE LIGHT, however I had the CHECK GAUGES LIGHT on the cluster, but after replacing the pick-up coil the light did not come back on for the 10 minutes the car was running with about 4 minutes in drive

Mike:

Read a few forums on here and thought to check the injectors for ohm's

Somewhat worried, I thought 12ohms was considered low and that there shouldn't be more than a 0.5 difference


1 5.5
3 14.5
5 5.7
7 5.6

2 17.6
4 17.0
6 6.5
8 8.7

Car starts and idles fine when cold, idle drops in drive after a few minutes, 1/4 mile or so.


Parts Replaced in the last week/ 5 miles
Distributor cap and rotor
Ignition pick-up coil and ignition module(significant improvement after new module)
Spark plugs and wires

Parts in Limbo
Ignition coil to complete 160,000 mile tuneup making it a new distributor
Fuel Injectors

Mike:

I know I need injectors but would a faulty ignition coil destroy injectors or is it more likely poor fuel or ethanol deterioration. Thanks

Mike:

Parts Replaced in the last week/ 5 miles
Distributor cap and rotor
Ignition pick-up coil and ignition module(significant improvement after new module)
Spark plugs and wires
Ignition coil
Thermostat and upper radiator hose


Car will start up and drive fine idle at 850, tires burning and all, turn the car off for about an hour. Come back to it, start it up and immediately in drive the idle drops to 500, drive for about a mile, throttle response is non-existent put back into park and in less than 5 minutes it died. I have replaced everything in the distributor, noticed a bulging radiator hose last night and replaced that hose as well as the thermostat for good measure. I'm at a loss for ideas, I know it seems like I'm just throwing parts at it but those all had their share of mileage so it was done as a 160k mileage tune-up to rule out parts.

jgalfo

mu understanding with the resistance for the fuel injectors that anything under 16 is bad.

I replaced mine with ones from FIC and it is a different car. runs and starts 100% better

John

Mike:

I understand that injectors would be a significant improvement but would low resistance injectors cause the engine to not start after it warmed up, display poor throttle response and cause hesitation at dead stop in drive

aklim

IF the injectors are bad, it can cause all sorts of problems since the ECM is trying to adjust and might not be able to adjust it. I would send the injectors to Jon and have him test and clean them. That way you KNOW for sure if the injectors are the issue.

Have you reset the idle speed? IF so, how? Is the TB clean? Are there any air leaks?

Mike:

The TPS failed the diagnostic check. 0.3 volts closed throttle and 3.8v full throttle.
New TPS from Bennett Auto, Ac delco, .67V closed and 4.3V still having the same problem. I know it should be .5V and 5V but no adjustment can be made to decrease closed throttle while increasing full throttle voltage. Drove it around the block, idle is still unresponsive, no throttle acceleration and idle drops to 400rpm in drive, raises back to 750rpm in park. I did not want to adjust idle speed but I will and I'll report back tomorrow

Paul Workman

The TPS is toast. Contact Jason Francis at Vette2Vette (815-673-6200 or ...6201)

RE "refubished" injectors, there have been several instances where they have failed in short order, in LT5 applications. However, the new stainless injectors from FIC have been "golden". I couldn't believe how much better my LT5 idles now!

Something else... low resistance is indicative of poor fuel delivery. A poor injector in my LT5 resulted in a burnt exhaust valve. (Pulling the head on an L98 or LTx is bad enough. But, that is serious PIA on an LT5, as the engine has to be pulled, the valve replaced, the cams re-timed, and then re-assembled and installed!).

I guess I'm saying, don't phuch around with bad injectors...or "rebuilt" ones either, for that matter!

P.

Mike:

So my new TPS that is 0.67V closed and 4.3V open is bad or are you referring to the old TPS.

I called FIC and the delphi stainless steel injectors were highly recommended for stock application on an L98 but a quick look on summitt lists name brand injectors, Accel, for under $300. I'm just interested in getting the vehicle running and keeping it stock for now with money possibly going to an ls3 swap in the future.


Has anyone used Accel fuel injectors?