Hi, I have a 1990 Corvette with a stock L98 engine (125,000 miles) with a slightly sloppy idle that I have tried to make smoother. I had the heads off 5 years ago because of a leaking head gasket so I took the opportunity to freshen things up at that time. When looking at the timing with a strobe light, the indicated position seems to wander more than I would expect, maybe 3 degrees. I am thinking this may be the cause of my sloppy idle.

Worn?

More Gear Wear

Less Gear Wear


The refresh included: plugs, wires, cap/rotor, fuel pump, fuel filter, fuel pressure regulator, air filter, fuel injectors, valve job, cleaning of the throttle body, re-bushing the throttle body, and likely a few other things I forgot. The engine ran the same before and after my repairs, minus the coolant leak.

I am currently working on resurrecting dad's old 1952 MGTD and it has a badly worn distributor. The timing on that car is a mess, but the drive gear is fine. It got me thinking bout my Corvette and I remembered I had taken a video a few years back when I shimmed the end float to about 0.012". I didn't think this would help, but I wanted to inspect the internal gear and see if there was anything wrong. Looking back at my video from 2021 there is visible wear on the Vett's drive gear, but I don't know if this excessive, or normal for 125,000 miles.

I have attached a few videos to help with your evaluation. What do you think, is my gear worn out

 

That doesn't look terrible to me. In fact, it's about on par with how much play my rotor has. That said, it wouldn't hurt to put in a new gear.

But I also don't think the 3 deg of timing variation is the cause of your idle issue (my guess is you measured that with the EST connector disconnected?). When the EST is hooked up, the ECM itself can actually vary the timing up to 5 deg in either direction to help maintain the correct idle speed, and it does that without the driver even being aware of it.

What gives you the impression there's actually an issue vs this being normal for the car? Was it smoother before?

For the age of the car, one thing you might want to look into is the pick up coil under the rotor. I found it can have a profound effect on how the engine runs.

https://www.corvetteforum.com/forums...l-for-hei.html

 

To answer a few questions:

I checked the timing/spark advance with the EST wire disconnected.

This car has always had the same idle, when I bought it, and when I did all the work to it.

I am comparing the idle smoothness to other V8 cars I have had over the years. It "seems" that it should be smoother.

I might look into a new pickup coil, thanks.

Regards,
Tim

 

How much idle vacuum do you get? And at what rpm? If automatic, in gear and in neutral?

Keep in mind too, these TPI systems are batch fire (all injectors fire at the same time). In terms of idle air/fuel distribution to the cylinders, it's not going to be as efficient as a sequential injection system, where the injector firings are timed to the cylinder firings.

Also, you're 100% sure the cam is stock? Previous owner didn't do any mods to it?

 

The gear looks fine. If you are concerned, since you have the distributor out already, it takes only a couple minutes and a few dollars to pop out the roll pin and put a new gear in.

 

It is a 6 speed. So idle is around 700, I think.

I think I am the third owner. The guy I bought it off never did any upgrades to the engine. I have never seen any evidence of the engine being worked on until I put my hands on it, everything looked stock. I would be VERY surprised if it had a different cam.

I don't remember the vacuum at idle, but it was steady. What do you think it should be? I can check it...

I have extensively checked it for vacuum leaks, including a smoke generator. None were ever found any, except for maybe a small amount at the throttle body, and that has since been re-bushed, to no benefit.

Thanks,
Tim in Mass

 

I'd probably expect about 19 inHg out of a stock engine I'm neutral at that rpm. It's kind of a proxy indicator for idle quality

 

So, I just hooked up a vac gauge and I see around 19" at 900 rpm.

You can see/hear the sloppy idle in the movie.

-Tim in Mass

 

To be honest this all looks really normal to me for a car of that vintage. That vacuum reading is actually quite stable IMO. Barely fluctuating a 1/2" of Hg.

 

It sounds like it may have a 'light-hitter' (anyone that's racked a Detroit Diesel 2-stroke knows the term) cyl caused by a low flow fuel injector.

Here's how I check them. Not perfect, but close enough to find a weak injector:
You need either a loose injector connector pigtail (best way) or a couple of small probes you can touch the pins in the injector with. One pin will be connected to +12V, the other will be temporarily touched to ground. (I use my remote starter button for this.)

Install your fuel pressure gauge on the rail.
Unplug all 8 injectors.
Run the fuel pump, you can use the key for the 2 second prime. << Easiest way.
After the pump runs, let the pressure stabilize, and note what it is.
Energize the single injector under test by grounding the second terminal. I count "1 - 2 - 3 - 4" and remove the ground. Note the pressure drop for that injector.
Repeat for the other 7 injectors. You have to run the pump again for each test. Starting pressure should be really close to the same for each injector.

You're looking for a trend for the pressure drop. If one is not flowing like its siblings, the lesser pressure drop will stand out.

Its not tough to do, and doesn't require any special tools or knowledge.

 

Hi,

I like the idea of the injector test, but I don't think that is where my trouble is.

This engine idled the same before I changed the injectors. It is possible that one of my rebuilt injectors was/is bad, but unlikely I think.

-Tim in Mass

 

What injectors did you get?

 

The injectors are 22 pound Bosch from Fuel Injector Connection.

 

It's possible you have an issue with the injector voltage offsets for those injectors vs what's in the ECM tune. Sometimes Bosch-Iii Injectors have that issue.

If the car runs good overall then that may not be the case.

But it could be possible you'd need to have the ECM retuned for them. What part number is on the injector bodies? Should start with a 0280. Or 280.