I have been chasing a phantom in my 96 C4, with the LT-1 engine, for over a year. The engine will start up immediately when cold. However it has no power and there is a slight miss. (I have a protégée with more acceleration) And miss may not be the correct term. The tech 2 shows no misfires at all. It's like a rhythmic bump or roughness. You can put your hand on the intake and feel it. It's a very slight something. Once the engine warms up and goes to closed loop the engine will not easily restart. Almost like a flooding condition where I have to press down on the accelerator to get the engine to start. While running I do not hear any valves tapping. The engine is relatively quiet.

When I have time and money, I've replaced all sensors, coil, fuel injectors the wiring harness and the computer. Absolutely no change. I've tried replacing the opti spark (3 Times), wires and plugs and no difference. Where possible I replaced with GM parts or a reputable brand, although that is difficult with the opti spark. Many of the parts came from Zip, Ecklers or Summit.

In open loop my o2 sensors in the front are around 100 and the rear around 50. Closed loop they go up to 450 and fluctuate all over the place. I'm not receiving any trouble codes. I haven't been able to get the screw in compression test threaded to any cylinders but I was able to use the push compression tester and compression seems ok. I've gone through with a noid light and all fuel injectors are firing. There was a slight change in engine sound when I unplugged each fuel injector. I have a steady 40psi fuel pressure, 50 psi with the pressure relief vacuum removed. All Plugs are firing. I've triple checked that the wires are correct. I'm still trying to figure out how to upload the tech 2 snap shot to the computer or I would post it.

If any one on the forum has a suggestion, or test or something I can try, I'd really appreciate it. If not I'm replacing the fuel injection with a quadrajet and the Opti with an HEI. I want to drive the car not look at it in the garage.

 

Have you done a leak down or compression test? Id start there and rule it out 1st
.

 

I have been unable to thread the screw in compression tester into the cylinders. I have tried one of the push in compression tester. Not as accurate but the compression is in the general area. I'm not sure what a leak down test is but I replaced the intake gaskets, and the engine vacuum seems good. No water in the oil or any contamination.

 

Leak down is pumping air into cylinder to see % of leakage. Goes hand in hand with a compression test.
A vacuum gauge will tell a bigger picture as well. Get a cheap one and hook it up. Make sure its steady at idle. A vac gauge can tell you any number of 9 or so problems.
Always follow the KISS rule, and make sure she mechanically solid before making yourself crazy trying to figure it out.

I re-read your post again.....Plugged cats maybe??? Explains the lack of power, and the cold/hot start issues.

 

Yes, I have done vacuum checks and they read fine.

 

I know you're a DIYer, but If all else fails you could take it to a reputable mechanic familiar with LT1s and get it diagnosed, whether you have them fix it or not.

 

I'm not a DIYer by choice. I haven't found a mechanic in the area experienced with the LT-1 let alone willing to work on it. Most repair shops in the area are looking for the quick turn around. This is an issue that will take a little digging and time. The local dealership flat out refused to look at it. The excuse, they couldn't find the old computer in the store room they used to work on the LT-1. I'm desperate for any help I can get. I'm at the point I'm about to throw a carburetor and HEI on it and toss the opti in the trash, just so I can drive it.

 

FSM diagnostics are based on having GM scanner. Using spark tester is there spark during extended crank condition? What are ST and LT fuel trims? Are you familiar with the special function menu of Tech2? Local corvette, f body, impala ss, or Sports car club may have referral to someone to help you.

 

I will find out the ST and LT tonight when I get home. The only club in my area is a Miata club that runs on the local track. I will dig in to the special function menu tonight. I live in an area where is you have any skill at what you do, you hop on a train and work in DC or northern VA. This includes Mechanics. I'll look around for different clubs.

Thank You

 

Last night I played with the scanner special functions. I was noticing the DTC section where it lists all the trouble codes as either passed or failed. Most were passed. I did find two P0118 and P1115 Engine coolant temp sensor circuit too high. The temp sensors are brand new but I need to check them. I also notice one that would flash fail then pass. p0200. Injector control circuit fail I set the scanner to take a snap shot for that specific DTC. When reviewing the Snapshot on the computer, many frames the p0200 was pass then one frame fail. Over the course of the 30 second snap it occurred at least a dozen times. I also noticed a P1222 code at the same time. Intermittent Injector Control circuit fail. Neither set the check engine light.

I have previously replaced the injectors with a GM Reman set I purchased from ZIP. They made no change to the problem. The stutter was there before the change and after. I'm thinking it is a wiring issue or computer. I do have an extra computer that I'll swap out tonight. and try that first. From what I've seen on the diagrams, The same two wires control the four injectors on the same side. Two wires control the four even injectors and two wires control the four odd injectors. Brown 10 controls the even injectors and Brown eleven controls the odd injectors. In theory I should be able to continuity check the four ground wires to themselves and the other four wires to Brown 10 or Brown 11. I've used a noid light and they all fire so I'm guessing that I'm looking for a short or a break.

Does anyone know what the two fuel injector wires are or do? From what I have seen there is only two wires coming from the computer to the fuel injectors. Is one a ground and one positive? Where the computer flashes the positive? Or does the injector receive ground from the engine and one wire is positive and one is a signal? Any suggestions on other tests?

 

The 12v to injectors is fused. The fuel system in 94-97 is sequencal so each injector has a ground circuit to PCM. Each injector is pulsed once following the firing order.

 

@jcooperman59 What happened when you swapped computers? Did that fix your problem or did it introduce any new ones? Are you getting any different codes with the new computer? Would be interested to hear an update.

 

I have a spare wiring harness that I ran continuity checks on so that I could map where each fuel injector wire traced to on the computer connector. With that knowledge I started to check the actual wiring harness in the car. I checked the positive side of each clip and measured about 10.2 volts. I checked the resistance on each fuel injector and found them identical and within tolerance with a set I had on the shelf. So far so good. I next checked the continuity between the red plug on the computer and each fuel injector. All matched except the number 3 wire. I was unable to find numbers on the plug so I made my own. Clip down, left to right top and bottom. On the spare wire harness the number 3 cylinder fuel injector wire traced to the number 5 pin on the top of the red plug. In the car the number 3 cylinder plug traced to the #2 pin on the bottom of the red plug. Now when I listen to the engine I don't notice a difference between when the #3 cylinder fuel injector is connected and when it isn't.

I will try Hardway's suggestion this evening. Swap computers and check the results. I'll do a snap shot on each computer and let you know the results. I need to trace the rest of the wires on the red harness and see where they go. If possible I may just swap out the two harnesses.

I didn't buy the car new. I have no idea what was done prior to buying it and the car lot I bought it at is no longer in business. It did come with a trunk load of spare parts so... I had a feeling this was going to be a very involved fix.

Questions:
Can the PCM be programmed to pulse the fuel injectors on different pins?
Is there a way to pull the wire out and move it to a different pin location?

 

Terminal can be removed and replaced within connector. You will need to remove terminal lock on connector. Terminal tool is suggested ,IDK which one there's a bunch, sometimes a pick tool will work.

 

So pin 2 and pin 18 on the red connector are ground. I found a diagram on google. I was able to trace the pins on the spare wire harness the terminal that bolts to the block. I don't think I have a short condition as I would still have some sort of continuity on terminal 22. The only time that wire should see ground is when the PCM fires on that pin. This weekend I will pull the fuel injector wire all the way back to the PCM connector. I Youtubed the how to change a wire and will look for the piks on line. I'll check for breaks and shorts first and if necessary solder in a new wire. Fingers crossed

Than You for all the help.