1987 Corvette with 147k

Hey guys,
I have been working on replacing my intake manifold gasket for the last few months. It is finally warm and I got it all put back together, however, I now have new issues.

My car would not idle, it would just immediately stall out upon cranking. I have done lots of research and so far I have changed the EGR and the idle air control valve. The EGR would not hold vacuum and the idle air control valve looked pretty rough and was cheap.

Now I can get the car to idle, but only at about 1500rpm. It sounds very rough at this speed as well. I have true dual exhaust and I noticed that the driver side bank is running much smoother than the passenger side bank of cylinders.

I have verified that all vacuum lines hold pressure by replacing many of them. I have done my best to check all electrical connections, although I wouldn't be surprised if one of those is the culprit.

I checked my CEL and it is a code 12, but I could not figure out what that meant. MAF seems to be functioning properly, but I do want to replace the MAF relay. The spark plugs are black signifying that she's running rich, which I'd agree with. They appear to have been changed recently though.

Any ideas or suggestions would be very much appreciated! I gotta this thing on the road before spring is over!!

Thanks,
Ryan

 

Code 12 is not an error code, if that's the only thing you got, there are no stored codes. Did your idle problem start after you did the repair? Quite often a malfunction that appears immediately after work has been done is related to the repair work.

 

Well as for the repair, here is a little history:

First weekend: I took off everything down the manifold and 3 bolts stripped out. I decided to reassemble because I didn't see a free weekend ahead to finish up. It ran fine while continuing to leak this time.

Second attempt: The car was apart for a long time during this repair because it took a long time to get those bolts out and retap and reassemble. Once the bolts were in good shape, I did a shoddy repair job with the gasket and my coolant leak went from being external to being internal.

To go along with the internal coolant leak, I also experienced a no cold start condition. I had to baby the throttle until the car warmed up and then it would idle like normal.

Third attempt: I did it right this time and was hoping that upon reassembly, the cold start conditions would go away. That leads me to where I am now.

I've had the car apart for months in all and I'd go as far to say that I think it is just like it was when I pulled it apart the first time.

I tried unplugging the MAF sensor and it gave me codes 33, 34, and 36. I cleared the codes and ran the car and that's when I saw the code 12. That's good I guess.

I have run it since then for a while and I'll re-check the codes tomorrow. Maybe something new has popped up.

 

After I swapped the intake in my 86' with a TPIS unit, I had a similar "High idle, runs rough" condition. It turned out to be numberous vacuum leaks.

This is how I check for vacuum leaks:

1) Find someone who smokes
2) Acquire 4-6 feet of clear plastic tubing
3) Remove the snorkle from the throttle body
4) Slip one end of the tubing under the throttle plates
5) Duct tape the crap out of the throttle body to make it as air tight as possible
6) Have the smoker exhale smoke into the tube. It will take a couple of minutes to fill the intake. Use a flashlight to search for leaks, and have the smoker modulate their breathing to pinpoint the leak.

 

Well I have yet to check for leaks just yet because I feel pretty confident about the vacuum system.

In addition to replacing the EGR and IAC valve, I changed the throttle position sensor today. I didn't realize that I could adjust it by tilting it one way or the other. I thought that because my car seemed to be getting too much fuel, I would adjust the TPS such that it would send a signal of not getting much air.

This did not help, and in fact, the car ran better when I adjusted the TPS the exact opposite direction. I turned it as far counterclockwise as it would go, so the TPS arm is pressed the furthest it will go by adjustment.

I also wanted to mention that I do not have catalytic converters on this vehicle. I also did not have any engine codes at all today when I was playing with the throttle keeping it running.

My gut is telling me to look at the fuel injectors next. It still seems like the passenger side bank is running a lot rougher, and is keeping it from running.

I also want to thoroughly clean the throttle body, because I neglected doing this when I did the intake manifold gasket swap.

I also haven't pinpointed the timing yet, but whenever I make a change, I try running the car at different timing adjustments. Still haven't hit the sweet spot yet.


Any ideas or recommendations guys???

Thanks in advance!!

 

Are you following the correct procedures for setting timing, setting idle, and setting the TPS voltage? What is the fuel pressure as measured at the Schroeder valve on the passenger side fuel rail? Pressure with the ignition ON but the engine not running? Pressure with the engine idling?

Unless you do all of these properly and not using the necessary tools, it will be very difficult to get the car to idle much less run right.

The ECM needs to see all of the proper inputs so that it can make the adjustments to keep the engine running properly. Incorrect timing adjustment, wrong TPS voltage, and not setting the idle properly will cause the ECM fits.