Hello! I bought a 1990 Torch Red coupe about a month ago, and it has some issues that I would like to learn about before I start working on it over the winter.

116k miles, rebuilt transmission, Z06 rims.
Almost consistently, while idling, I can feel that the engine feels like it's "kicking around" between intervals of .5-2 seconds. According to the previous owner, the car has always had a rough idle throughout the 10 years he has owned it. I wouldn't be too concerned about it, but there's another issue that I believe could be causing it.

About 30 minutes into the drive home from buying the car, the engine service soon light pops on. Code 32 (Exhaust Gas Re-circulation). At first, the light only triggered after about 20-30 miles of highway driving, but now it's down to 10-15. There's no noticeable difference in how it drives after the light turns on, and the idle is the same mild roughness after 40 miles as it is when I turn the engine over.

I did some reading, and I think I've narrowed it down to the EGR valve itself. The car has a consistently shaky idle, whether the engine is hot or cold. Do you guys think that's a good place to start? And since I'm going to be in that area anyways, I'm going to install a new EGR temperature switch just in case. Only issue is that I have only light mechanical experience. Is there anything I need to know before diving in? I'm going to have to take off the intake plenum in order to get to the valve, and I haven't found a guide to this specific process online so I'm just going to go in blind. I don't think it'll be too hard though, should I be worried?

 

If you are there, change the fuel pressure regulator. Before all that, see what injectors you have.

 

Bosch IIIs have been known to cause idle issues with TPI.
Have you taken a fuel pressure reading and done a leak down test?

It's a new car to you. I would suggest
1. Pulling and replacing the plugs
2. What condition are the wires in? I would suggest replacing them if there is even a hint of cracking or any gouges.
3. What is the base timing set at?
4. What fuel pressure are you seeing Vacuum line on/off? Do a leak down test
5. What RPM do you idle at in P and in D?
6. How old is the fuel filer?
7. Condition of Air Filter?
8. Is the Throttle body and IAC housing clean?
9. Do the minimum idle set procedure
10. Inspect all vacuum lines - check routing to diagram posted on your car. Make sure everythign is hooked up correctly.
11. Coil/Cap/Rotor condition and age?


Once all the general health and maintenance items have been done...

Takign the plenum off is as simple as pulling all your connections and vacuum lines (I suggest you put a piece of tape on the ends and mark where they go), disconnect throttle cables (etc.) from throttle body, then take the torx headed bolts out of the connections between the upper plenum and runners. You will gently pry on it and wiggle it, but it will come out. The general health items probably won't fix the EGR code, but It's all good activity to prevent other issues and know you are starting from solid ground for troubleshooting in the future. The idle stumble and EGR may not be connected.

 

1. Previous owner said he put new spark plugs in, but I'll still check
2. Which wires specifically? They all look to be in pretty good condition.
3. How do I find the base timing? I don't know what that is
4. I don't know what either of those are and I don't know how to check.
5.1,100 in neutral and 900 in D
6.No idea, would I have to pull the filter to check or is there a way to check the condition by looking at it?
7.Clean
8. Throttle body appears clean, I don't know where the IAC housing is though.
9. What is the minimum Idle set procedure?
10. All are good to my knowledge.
11. I don't know what the cap/rotor is referring to, but the ignition coils appear to be good.

Will I need to remove the throttle body to remove the intake plenum?

As you can tell I still have a lot to learn... Thank you

 

Pick the worst one to get to, get it out, and if it's new(ish) he probably is telling you the truth. However, I hope they are the right ones as well. Probably should look the number up just to be certain.
I was referring to the spark plug wires. If they look good just have a look under the hood while it is dark and see if you have any arcs (engine running).
There is a brown wire you will need to locate that has a disconnect in it(ESC Wire). I am not sure where it is on a Corvette, my C4 is an LT4... but all L98's have this. This is the signal wire from the ECM that controls timing. You disconnect it and the only timing you have is the distributor advance. This is the "base" timing. Should be 6* BTDC. Plenty of references to look up to see how to set timing on a Chevy Small Block. You will get a Code 42 when you disconnect the wire. Plug it back in, disconnect the battery to clear.
There is a schrader valve on the fuel rail, has a cap on it, just like your tires. You can connect a gage here to see pressure in the rails. You will also see what looks like a top hat under the plenum at the rear passenger side of the rail - this is the fuel pressure regulator. There is a vacuum line that goes onto it. When vacuum is applied fuel pressure goes down (it lets more fuel return to tank). With the line off you have no vacuum... this is you max fuel pressure. Should be at 43.5psi (Engine Running).
That is HIGH. When the engine is warm you should see about 700-750RPM in D... usually about 850 or so in P. refer to #9.
Unfortunately no, but if you do #4 and have good pressure it's OK, but as a PM you should replace the fuel filter. Its cheap and can cause fuel delivery problems. I change mine about every 2 years on my "fun" cars that don't get much mileage.

The IAC pokes out to the passenger side of the throttle body, under the opening. It is pretty big in diameter to any other sensors in the area. It is a stepper motor that controls the air flow past the throttle blades at idle - IAC is Idle Air Control. You will notice if you take your intak ducting off the throttle body you have the two round throttle bores and in between them a little triangular area. This is the area ait can move through a cavity around the throttle bores and into the plenum, The motor controls a pentile that moves to control the amount of air passing through to regulate idle. For it to work properly, this passage and housing need to be clean.
Google is your friend... just google "TPI Minimum Idle Set" or "TPI IAC adjustment" for a detailed explanation. Also, links to Camaro sites are OK... the TPI system in both works the same. What you will do is force the IAC closed, disconnect the ESC wire mentioned above, set base timing, set your minimum RPM with he throttle screw, then plug all back in and give the computer control again. With your idle RPM where it is, I suggest you do this soon.

Coil feeds the distributor cap, rotor rotates inside the cap.

Nope, you can leave it bolted to the upper plenum.

I am saying all this because I firmly believe you have to have a solid base to start diagnostics from and to prevent other issues from cropping up. When I brought my L98 IROC home I did all the above before I started jumping into other issues. It help with the overall drive quality. Your EGR code is probably because your EGR is failing.

 

Guess I'll have a busy weekend, thank you!

 

If you plan on keeping and enjoying this car for any length of time. I also suggest you get a laptop, TunerPro RT, and a USB to ALDL cable to data log and read live data.

 

1. I always assume the PO is either a lair, cheap or both. Usually works out. Get what you know to be good plugs and replace. You don't know if he used the right plugs, best plugs or plugs that physically fit.

2. Dark garage, engine running and spray a fine mist of water around the wires and see if they leak.

5. How do you know how many RPM? The tach? It has been known to be less than accurate.

9. I agree it has to be done but Google usually shows the FSM method which will ASSUME that everything is correct except the idle rate is wrong. Learn to use a scan tool to see what the IAC counts are. You can also see if the tach is right or what the other sensors are saying.

 

i agree 100%. That's why I say get the hardest plug first. If it has been changed then most likely the rest have as well.

Yes, I set mine by IAC counts as well, but it's not a requirement. It helps to validate, and why I said the same, get Tuner Pro set up. Its cheap to get going, if you have a laptop around, all you need is an $85 cable. It does what you need for almost everything. It will save you more in parts later than it costs. Very good investment. Even if you use a meter under the hood on sensors, it's still good to see what the ECM sees as well.

I also agree on the tach, again great to see what the ECM sees. Of course, you can just use you ears for a gross estimate in this situation. Does it sound like it is at 1000RPMs?

 

What is recommended these days for replacement on a factory engine?

 

No. I was at the parts store last year and I heard this guy ask the store guy for the right fluid. Of course, he bought the cheapest. He was recommended a filter and he said "No. I am selling the car and nobody will know if I changed the trans filter or not.". Just shop at the parts store and pretend to be looking while you are listening. You may be afraid when you buy a used car and they say "We just changed......".

I learned to do that when I could not set my idle the usual way. Found out that there was an air leak because the IAC counts were always 0 even though I just closed the throttle body till the screw could not come out any more. Very handy for diagnosing air leaks too.

My tach was weird. 300-400 above whatever ECM sees. I verified ECM reading with a Snap On timing light with a tach. Sent the cluster out for a rebuild and swap the tach with an 8000 rpm one that was made accurate and it now agrees with the ECM and the timing light. What is weird is that my 91 F-body tach was always spot on.

 

Bosch III's tend to be preferred. It also seems they do "OK" for Corvette guys.
The Camaro guys, well that is a bit different. Though, I have them in my L98 IROC.

Here is what is known, the Voltage offsets (Latency) is different. It can affect performance and idle quality. The Aluminum headed L98 would appear from the boards to have less issues. However, there seems to be less people data logging and trying to get BLMs in their proper place in the Y-body than F-body.

Delphi's are known to be direct drop ins and less noisy.

Any properly sized injector should work, but you have to make sure they are the proper flow rating at 43.5PSI or install an AFPR to run them at the proper pressure to have the correct flow rating in your car.

 

So with no. 4, I check the pressure in the fuel rails with the car running, correct? And could you elaborate more on the process of checking the max fuel pressure? Where do I attach the pressure gauge to the top hat thing? Do I attach it to the disconnected vacuum line or the connector where the line attaches to? Sorry if my questions aren't clear, I just don't fully understand the second part of your instructions.

Also, what specific ALDL cable would you recommend? I'm not sure which would be compatible with my car.

 

There is a schraeder valve on the fuel rail. Turn key to run and see what pressure you get. Turn key off and see if it holds. For Wide Open, drive the car and stomp on the pedal and as it moves, see what the pressure goes to. Helpful to have an assistant.

As the the ALDL, 1320 Electronics and an old Android phone.

 

How will I be able to read the pressure gauge attached to the engine while the car is in motion? Or do you just mean revving the engine in park?

 

5000 rpm in park doesn't burn as much fuel as 5000 rpm while in motion. You tape the gauge to the windshield.

 

Buy one with a long hose, tape it to the windshield, have someone ride with you. Been there, done that too.

What I do for a Fuel Pressure Test is, Key on, engine off... let it build for 2 seconds and see what the reading is. Write it down.
Start Engine, record fuel pressure
Remove vacuum line from Fuel Pressure Regulator (Top Hat looking piece attached to the fuel rail). Write down the reading.
Turn engine off, put vacuum line on (so you don't forget later). Write down the gage reading every 5 minutes up to about 30 minutes.

This checks the health of all components in your fuel system to supply proper fueling and seals. If after 30 minutes you haven't dropped below 20psi, all is healthy.

Now take it for a ride. You should see around mid 30s for Fuel pressure at idle, a little higher is ok. When you step on the gas, the pressure should rise into the 40-45psi rage and stay in that range until you let off the throttle.

You can't reproduce load and vacuum of wot without the load of moving the car (or on a dyno, but who has that at home?). This is very important because the old injectors on these cars are becoming a common failure at this age. A leaking injector or poor fuel delivery can cause a poor idle.

I am all about to free/cheap easy checks to eliminate potential issues before diving in deep.

 

It's been a while since I messed with a TPI. I'm pulling the intake off an polishing everything and I see Bosch III's going fo $50 on ebay and scratch my head. Are the factory Multecs worth rebuilding or just get the Bosch and be done with it? I have an 87.

 

The Bosch IIIs in my car are "OK". Idle is a bit rough and BLMs are high.
If I did it again I would use Delphi's
Mine were installed by the previous owner. The Bosch injectors are cheaper than Delphi's and I think that is why everyone uses them. They are a perfectly fine injector, just have some different operating parameters that the stockers. This can be addressed in a Tune.

 

Besides originality, Multecs are good targets. You are fine if you chain yourself to pure gas or the ethanol will eat at the windingz.