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LT1 troubles, I need your help


Well-known member
Jul 3, 2001
Auburndale, Florida
1969 Killer Shark
Okay, this might better belong under "other cars", but it should get more direct help here.

I installed a 1995 LT1 in a 1936 Ford using the OBDII computer, fully electronic trans with a Street and Performance harness. The car is not using a Mass Air Flow, and the computer has been reprogrammed for the 3.73 gears and power tuning.

I installed a new Optispark, coil, wires, plugs, O2 sensors, injectors, and temp sensors for the air intake. It has a nice new high pressure pump in the custom tank and proper fuel pressure at the rail.

Here is the deal. It either idles too high all by itself, and or loads up on fuel. It is very rich at idle and will make your clothes stink standing around it. On the road, it is a rocket. One of the fastest 0-60 cars I have ever driven. The trans downshifts, upshifts, etc. just fine. However, it will pop, pop, like a lean condition or bad spark when you get on it after running at slow speeds.

I have had no luck getting any G.M. tech people to log into the puter.

So, have any of you 92-96 guys had problems with your idle, popping back, and loading up on rich fuel? What was the solution?
Hi there,
You will need to post your fuel pressure, and are you sure on the OBD2 computer, because if your that rich, you check engine light should come on????
How many o2 sensors did you have to put on the car??
Also, are you using a return line for the fuel rail??
Please let us know, and we can proceed.
Besttoyou, c4c5
c4c5specialist said:
Hi there,
You will need to post your fuel pressure, and are you sure on the OBD2 computer, because if your that rich, you check engine light should come on????
How many o2 sensors did you have to put on the car??
Also, are you using a return line for the fuel rail??
Please let us know, and we can proceed.
Besttoyou, c4c5

Fuel pressure at max around 50. It drops down around 30-40 with the vacuum on at idle.

There is no check engine light. This is at the advice of Street and Performance. Because the OBDII is so sophisticated and this system is only running the minimum including no Mass Air Flow, it would glow all the time. They suggest not even bothering with it. I have hooked them up before on the first generation LT1 and tuned port conversion. Both of those can convert to Speed Density with little to no problems.

We are using a Rockvalley fuel tank with a built in pump with pressure and return line. Street and Performance supplied a beautiful fuel line kit to come off the rails and out the right rear.

I was told today that Street and Performance has had some problems with mislabled harneses at the O2. It is possible the right and left are mixed up. This would cause a major problem as the computer fires the injectors left and right depending among other thing from the signal at the O2.

The only three orginal parts include the regulator, TPS, IAC, and MAP. All other sensors, injectors, ignition, coil, etc. etc. are new.

Does it really sound like mixed up O2 wires could cause this kind of problem?
OBD1, not 2

Hi there,
Well, then you are OBD1, not 2. The reason is that all OBD2 in Corvette ran 2 sensors in front of cats, and 2 behind. I know you dont have cats, so this is not an issue. However, your tuning diagnosis is critical as the earlier units were batch fired, as you stated, one bank fires, then the others. Your fuel pressure should be 41-47 when you key up. And it should hold stable, and not drop. When it is running, you should see about 35-40.
As for if this would cause this condition, its possible. I would check for exhaust leakage before you o2 sensors first, as this would show you a lean condition, and the computer would add more fuel.
You can still use a 95 engine, however, you will not be running the OBD2 calibration. Tell me something, does your Optispark have the 2 vent tubes on it???? This could be a clue.
Also, how is diagnostics achieved??? I would say that your next course of action is a check of your data coming to the computer.
Also, if I could ask one other thing, to confirm wiring, and firing sequence, please tell me what colors the drivers side front injector, for the #1 cylinder, wiring harness is please.
With this information, we can proceed, please keep us posted, c4c5

It is a 12 Pin connector, dual O2 on a 94-95 style computer. I guess it is not considered OBDII. Sorry about that. The engine is a 1995, that I am sure of. Street and Performace set up a custom harness that deletes the Mass Air Flow. They also reprogrammed the computer to make it shift right and avoid the 98 mph speed limiter.

I borrowed a scanner and set it all up using either 94 or 95 with vin code P (8th) and F (4th).

It won't scan. They had the same problem at the chevy dealer. It would read at first and led them to believe that injectors were bad. That is why they replaced the injectors. Now, it won't even pick up. I am not aware of any bad grounds, have power at all the connections.

I can only assume the puter is shorted out??? The car went through three bad alternators right off the bat. One of them was spiking over 17 volts until it blew out. The body shop was jumping the car off.

I am lost. The owner of the car is getting ticked off, and I really don't know where to turn.
HI there,
Try this, do you have a check engine light?????
If so, ground pin A and pin B. You know that you are looking for a code 12, if you have that, the computer is going into diagnostics, and it will show you any codes.
Since you are programmed off an F body, you should be able to retrieve this way.
It will also give you an idea of the computer condition. As if your code 12 does not show, you have just failed the diagnostic circuit check.
Please let us know , c4c5
Check engine light is not hooked up (at the advice of Street and Performance as it would be lit all the time), but I hooked it up and can't get any action out of it (no 12 or anything).

I guess that is telling me the ECM may be dead.

I called Street and Performace and they are going to go ahead and ship another ECM with the system reprogrammed.

There is something interesting I have noticed. Nikki's 90 gets a weird Gremlin in it once in a while. Sometimes when you start the car it will idle about 2,000 RPM, run very rich, and the SYS will flash on the dash. No check engine light. The speedo won't register and it runs terrible. If you shut it down and restart, it will clear out and run fine. I hooked the scanner to her car for giggles and grins and it has not retained any historic memory of this event. In fact, the car does this about twice a month. Since she can shut down and restart it really has not been a big deal.

Fast forward to the 36 Ford. It idles high, rich, and will foul down the plugs. The computer won't respond. It has a good ground, power, and ignition input. All the lights are on, but nobody is home.

I can't figure out how the car would run at all if the computer is not responding. It seems to have started this after it had so many problems with the battery and charging system.

Thanks for your feedback.

I just went back and looked at the "check engine" light again. I hooked up a temporary light. It glows as soon as the key is switched on, while the engine is running, and goes out when the key is turned off. When I ground A&B it still stays lit and will not flash.

The ground is good, we have full power on the 12v constant and the 12V switched.

I guess the ECM is the most likely suspect.

Sound right?
I would agree with your diagnosis.
What I would do is have your vendor send also a complete wiring diagram, along with operational parameters on this computer, and its program.
Also, request from them their recommendations for a scan tool to communicate with the PCM.
This way, you have all your duck in a row, if this does NOT fix the issue.
Please keep us posted, as I am very interested as to what is going on.
Also, on your 90, you can access your CCM before it is shut down, you will most likely show a communications code. My guess, from what you say is that your communications system is being pulled down during this event, that is why it does not register.
Another tip on this, if you cannot communicate with your PCM, or the CCM. This is what has happened. While still having the vehicle on, see if you can communicate with your airbag system.
My guess is that you will be able to talk to your airbag, but not the other 2. Also, note the HVAC for any flashing LED.
Besttoyou, c4c5
Seems one of CAC's best resources, c4c5 specialist has this pretty well in hand but I'd like to offer some insight.

First, the PCM, if it's truly a 95, it's not full-OBD2. 95 VIN Ps only turned on the light for the "traditional" (at least they were back then) OBDI codes. OBD2 codes were stored, but did not turn on the light and did not affect the operation of the engine. 95 Y-car was part of GM's "early-roll-out" of OBD2 and the OBD2 codes were accessable only by scan tester. Any OBD2 DTC information was strictly a research project and dealers would forward OBD2 codes they found in 95s to GM. So, if the light is on--even your temporary light--then the problem is an OBD1 DTC, ie: basic stuff. This shop that told you to not connect the MIL because "OBD2 was too complex and would have the light on all the time" clearly lacks knowlege of GM diagnostics (ie: they don't know that OBD2 codes in 95 ECMs *don't* turn on the light) and probably does that to cut corners. In reality, the OBD1 diagnostics in a street rod are a good thing.

Fact is, those guys should be building these harnesses and whatever to use full GM diagnostics which would enable techs and DIY's to sucessfully diagnose problems like you are having. Doing it with hacked up harnesses and systems is just sloppy work.

Also, I'd tell them to put the MAF back into the system. The engine may drive a little better and using a MAF signal will allow you some leeway or "fudge factor" in calibration for future engine modifications. If you stick with speed dens. you're going to have to redo the cal. everytime you change something. While it is true that the 85-89 MAFs majorly sucked, the 94-up units are pretty reliable and durable.

The O2Ses being swapped in the harness--hmmm. I'd think that wouldn't make a large difference. I believe the O2S input is analog in nature and not discrete nor specific to the firing inpulses on a certain side of the engine. That has me believing that swapping the O2S inputs side-to-side while not correct would probably not create a big problem *if* the rest of the system were working right. Now, what might be wrong is a bad O2S or a poor O2S ground.

In re: with the temporary light in place, key on/ engine off and no DTC12, there's trouble somewhere, but with the hacked up wiring harness, I'd sure consider that in itself a potential problem. I suppose the ECM could also be a possible cause, but since you say the car only has trouble at idle and runs hard during acceleration and has a 4L60-E that works right, I'm really having a hard time believing the ECM is bad. Sounds to me like the ECM can't control idle and may be dropping out of closed loop as a result.

What kind of camshaft is in this motor?

What kind of exhaust and where are the O2Ses in the exhaust?

On the 90, check all the ECM and CCM grounds. Actually, the car's old enough that I'd remove and clean all the grounds for good measure.

Again, take these as added ideas after you check into all the issues c4c5specialist has suggested.
Thanks Hib.

All and all, Street and Performance does a pretty nice job of "hacking" up a harness and making something that will run in these street rods. I prefer the 92-93 LT-1 as the Computer can just be chipped and for some reason they are easier to run on speed density.

I am only going to repeat what I have been told. One of the main difference in 94-96 LT-1 engines is that they batch fire the injectors left to right in conjunction with readings for each side of the engine. All things being equal, it should not matter. If the engine is running in a hacked up open loop kind of mess to begin with and it the O2 wires are crossed, it may try to richen or lean the wrong side causing the problem to spiril out of control.

Here are two quick pics of the set up in this car. What you see here I have installed. There is NO room between the radiator and throttle body for a Mass Air Flow. The last one of these I installed I put in a 90 1500 Chevy truck and used the Mass Air Flow. I must have just gotten lucky because it ran like the wind off the bat and seems to have all the normal driveability creature comforts of the donor car it came from.

C4C5, Steet and Performance supplied me with a diagram of the harness, and just now a "mock" vin for diagnostic purposes. More or less, the 8th digit is P and the 4th digit is F. They really don't care what tool you use to scan the system.

Because this thing does not have a Mass Air Flow it will never run "perfect" in all situations and I have explained same to the owner. The engine is bone stock other than the headers. The 02 sensors are within inches of where they would be on a factory motor and they are the heated kind.

I am really hoping that the bad battery that blew the alternator spiking the system with crazy voltage did the damage.

C4C5, you are right about the 90. The CCM does set a code when it goes crazy like that saying that it has lost communication with the host. In the past I have cleaned the computer connectors. Still not sure if that intermittent problem is in the ECM or CCM, or just a loose wire.

Keep your fingers crossed for me. I have to get this thing out of my hair this coming week, and other than in a magazine or concourse show I hope to never see it again.



For some reason the photos are not blowing right into the thread right now, so here is the link. Check out the last two photos on this page for a close up on the fit between the radiator and the actual puter I am running.

Fixed it!!!!!!!!!

Okay, this is really weird. Got Street and Performance to burn another computer and overnight it here. I yanked the old one out. What did I find? There was water and heavy corrosion in one of the four harness plugs (the one that the diagnostic lead comes from). No wonder it would not run very well and lost its ability to scan.

I cleaned it up and installed the new puter. Runs great and the diagnostic circuit is perfect.

Now I have another problem. This was a used engine of course. After it gets good and warm the oil pressure at idle in drive drops to 9 psi. I put another gauge and sender unit on the car to verify and got the same results. The intake manifold has also sprung a leak at the rear where it seals to the block and is leaking oil out of there when it gets splashed around. In other words, it won't leak sitting in the driveway, but when you drive it around and come back to the shop it has a nice puddle running off and down the back of the engine.:hb

So one problem solved, more to come. The lack of oil pressure is scary, and the engine makes a lot of valve train noise. This was not noticeable when I first assembled the car because I was running open headers at the time, and after I put the exhaust on I could not drive at it did not have a front clip.

This is going to get very nasty.
HOld on

HI there,
Glad to hear that you got all your PCM problems done, however, I think I may have your issue as far as the oil leak, pressure, and the like.
When Corvette LT1 first came out, there was a bulletin issued for oil leaks at the back of the block.
92-304-6a is a bulletin in which they describe leaks from the rear CAMSHAFT plug, as it was not sealed correctly,causing leaks, which were misdiagnosed, as a rear main.
Now you say that you have low oil pressure, noisy valvetrain, and an oil leak that puddles up after a drive.
I know you are with me on this, so I will leave it at that.
Unless you PCV is totally clogged, I dont believe that it is your rear intake.
As a sideline, the correct plug is a GM part #3997752, per the bulletin.
If your intake is leaking, and they all do somewhat, #12346141 is a revised intake sealant, that cures in 24 hours. There was a bulletin issued on this as well.
Please keep me posted, c4c5:hbilovethisone

Thanks a ton.

I think I just discovered why it is leaking. The body shop installed a beautiful set of billet valve covers on the engine. I started looking at them today and realized that there is NO OIL BREATHER. They disconnected the stock breather tube that goes from the passenger cover into the throttle body and plugged the throttle body. Then, they put two oil filler caps.

That means that the engine is building up massive amounts of pressure that can't be relieve regardless of the PCV.

So, I cleaned the back of the intake and put a bead of the best quality sealant I could locate on there. I will get a regular chrome breather tomorrow, and let the sealant set for at least 24 hours.

Correct me if I am wrong, but would'nt a lack of a breather wreak havock on the engine seals?

Thanks again. I have made note of those bullitens and part numbers.

My guess is that the leakage and the low pressure are two different but simultaneous problems.

As the engine has low oil pressure but not no oil pressure, I'd look for an internal oil leak, a restriction in the oiling system near the pump, a problem with the pump itself or a restricted oil pump pickup.
Hi Chris,
Absolutely, you would have horrific pressures.
As for the rest, I think you are on your game. Hib has excellent points, that I would inspect for.
Besttoyou, c4c5:hb
Final report...........I hope.

The computer system is very happy right now (at least as happy as can be less a mass air flow sensor). The owner is aware that there will be some driveability issues due to the lack of the mass air flow.

The car is doing everything it is suppose to be doing, so BINGO, the corroded connector was the culprit.

As far as the oil leak goes, I did as mentioned before with cleaning the back of the block and putting the high quality sealer between the intake and block. I also bought a chrome breather and put it on the passenger valve cover..........leak solved. It is dry as a bone. So, the lack of a breather was pushing the oil out of the weakest link. Lucky it did not blow the front or rear main seal.

In so much as the oil pressure issue. That will continue to be a problem. I went ahead before changing the oil and put some Motor Honey in to see what would happen. It raised the oil pressure at temperature in gear about 2 psi, enough to keep the low oil pressure warning from flashing on the Dakota Digital Dash pod. It also reduced the valve train noise that seems to be the worst on the driver side. The engine has a fault, be it plugged pick up, or maybe that is why or how it ended up in the junk yard as a donor to start with. Like I said, I have tried two different oil gauges (both new) and two different sending units. It appears to be reading correct. The way this motor sits in the car you can't just pull the oil pan and have a look.

So, I will have a long talk with the owner about his options. At this point in time, knowing the electric gremlins are solved, I would suggest we go ahead and overhaul the motor, or at least talk to the salvage yard about a replacement.

Thanks for all of your help.
They still make the LT1 crate engine, and you could get it with a 3yr36k warranty.
I would suggest this course of action, as they are new.
I will post partnumbers, and list tomorrow.

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