I have a 1991 C4 TPI. I just replaced all injectors, EGR delete, fuel pressure reg, set TPS and IAC. At idle it has a slight rough idle. Any ideas of what I can check? Thanks in advance.

 

So...are we to believe changing components is substitute for troubleshooting? It's NOT...btw.

Suggest you start by datalogging to see what ALL of the sensors are reporting.

You replaced all injectors...why? Did you test them to see if their internal resistance was uniform? Were the at operating temperature during the test?

is the miss one (or two) cylinders or random? What do the plugs indicate overall?

What does the O2 sensors voltage and how many cross-over counts does each have?

What is the TB voltage (switch on, engine not running)?

etc.,,etc., etc.......

Throwing parts at it in hopes the problem goes away is very expensive and worse, it is rife with assumptions and opens the possibility of introducing multiple issues, further complicating resolution.

Suggest getting back to basics...

 

I like to replace things on my car before they go bad..so I wasn't just throwing parts at it...it has 97k on it so for reliability I replaced the injectors...all other parts were accessible when doing injectors...so why not. I think you call it preventative...

 

How did you do the EGR delete? Did you also get the MEMCAL reprogrammed?

How do you set the TPS on a 91? They are not set at all. Startup is considered baseline, IIRC. Mine come with no screw slots. Just screw holes.

How did you set the IAC? Using the FSM way? That makes a lot of assumptions. I'd rather use a scanner. Where did the injectors come from and what size?

As preventative, did you take off the TB, clean the IAC pintle, take of the IAC housing and the plate on top? Clean out all the passages while you are doing that and also do the TB Coolant Bypass mod.

How is the compression?

 

L98 are not known for having a smooth idle. I’ve had a few and all had a bit of a rough idle. It’s part of their charm. Lol

 

In stock form, it wasn't as smooth as a sedan but I don't remember it being rough when it was new and stock unless something was wrong.

Rough idles were a necessity in the past for power making cars. Thankfully today, it doesn't have to be. Today, we have more power and way better refinement.

 

Rough is probably not accurate. But L98s are not smooth idlers.

 

Sorry for the tone of some of the responses you've gotten here.

A couple of things to check off the list just to start:
1. Get the Factory Service Manual for the car.
2. Investing in a good scan tool is a smart move. I use an Actron 9690. Mine has been very helpful, and there's a lot of peace of mind being able to watch something like your O2 sensor doing what it's supposed to do in real time.

In the meantime:

Pull the codes, if there are any.

Check timing.

If the IAC and TPS are settled and you haven't done it yet, check your fuel pressure. My guess from what you have replaced is that you were chasing a fuel pressure problem.

Pull and inspect the plugs and plug wires. Pay careful attention to the plug wires. They can be broken internally and it's not always obvious.
On my car I carefully tested spark on each cylin… wait... that's a lie... no I didn't... I ripped those out and replaced them simply because they were old. In doing so, I found the problems they had.

Start looking at vacuum lines. Particularly around the area you were working. Scan tool will help HUUUGE with this because you can watch the IAC going nuts in real time if you have a bad vacuum leak.

***I took a look at a whole bunch of vacuum lines on mine and decided "welp, there's no point in a lot of them being there, just waiting to cause trouble" so I ripped the systems associated with them out. AIR, EVAC, EGR, etc... that's kind of a radical step, but anything I can do to simplify the system by getting rid of useless nonsense, I'm going to do. Depending on how insane your state is, that may or may not be legal.***


I am going through mine much the same way you are, replacing things from time to time merely as a preventative. I have very, very little patience for bothering to hem and haw about elegantly diagnosing something on a car this old. The sensors on my car are 35 years old and I'm not wasting my time pinning down which part I have to rip out to make the car run right when one of them is $9 and the other is $23. They're both gone, RIGHT FREAKING NOW, and then I don't have to worry about either of them for another 35 years, by which time I'll be dead anyway.

Like you, my fuel injectors will be replaced this winter, and I really don't give a damn which ones are or are not bad, and what the individual spray patterns look like, blah, blah, blah, BECAUSE ALL 9 OF THEM ARE 35 YEARS OLD.

They're gone.