Slightly rough idle and a fouled plug on a low mileage 1990

[Reichert]

Hi,

So I just took delivery of a 1990 with ~20k miles (here's my intro post: http://www.zr1.net/forum/showthread.php?t=28834). It has previously been driven at least monthly the last year and a half, for 20 or 30 minutes, only in normal power mode.

After several spirited drives of my own now, there are symptoms of a problem in the fuel system:

- Cold start and idle is beautiful. Runs great for a while.
- As the engine warms up, check engine light comes on, and then off again a little later. I pulled the codes and get only H65 (O2 sensor, right bank rich)
- At full operating temperature, idling at a standstill and especially with A/C on, it runs a little rough and causes the flywheel to start knocking a little bit (I am pretty sure this is just a symptom of the rough idle and not a problem in and of itself, but I could be wrong)
- I pulled the plugs, and #4 cylinder is dry fouled (picture below)

I replaced all of the spark plugs since that was easy, but no change in behavior. it still has the original plug wires stamped with "Corvette LT5". So far my best guess is a bad injector, and/or plug wire.

Before diving in and taking apart the intake plenum to diagnose further, I wanted to get your opinions and advice here.

2018-02-26_07-37-00

[tccrab]

If it has the original injectors, save yourself the trouble and just replace them before you dig any deeper.
It's a guarantee that they are failing or will fail soon.
Jerry's Gaskets will have the the plenum gaskets, and maybe even the injectors too.
There's lots of discussion about this, just do a search on fuel injectors.

[KILLSHOTS]

Sounds exactly like what mine did. Injectors.

[DRM500RUBYZR-1]

Originally Posted by tccrab

If it has the original injectors, save yourself the trouble and just replace them before you dig any deeper.
It's a guarantee that they are failing or will fail soon.
Jerry's Gaskets will have the the plenum gaskets, and maybe even the injectors too.
There's lots of discussion about this, just do a search on fuel injectors.

Marty

[Z51JEFF]

I’d pull the plenum and replace the injectors,maybe the plug wires and coil packs
and do a check of all the vacuum lines. If you can do the work yourself it’s no big deal. I work on my cars so when the injectors failed,@8000 original miles there was never any question about who would be working on the car. At the time I didn’t have the service manual yet so I came over here,asked a ton of questions and took a lot of pictures. There a few parts of the process that can work your neveres but not difficult at all. Plugging in the ignition module both front and back and getting the vacuum lines plugged in is a small challenge. Marc Haibeck sells a very detailed video of the plenum pull that is very helpful.

[jss06c6]

Sounds like when you go into closed loop and the O2 sensors are "engaged" to manage your fuel maps, the ECU is not delivering the proper amount of fuel since the input is wrong. I won't dispute the injectors being potential, but given that the car was rarely driven at decent speeds when fully warmed up and in closed loop, I'd take the easiest first step and replace both O2 sensors and recheck. Guessing that at least one of your O2 sensors is badly coked and reading incorrectly.

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[Reichert]

Thanks for the tips so far.

I drove it to work yesterday (~35 mile round trip) and then again in the evening, another 20 miles or so, and I swear it was running a lot better - it wasn't until I pulled in to the driveway in the evening that I could detect a little stumble, with the flywheel knocking just a tiny bit .. but not as bad as it was over the weekend. I pulled the #4 plug again in between those drives and there was no fouling (this was a new plug, so it might be that it just didn't have time to accumulate). I'm going to drive it for a few more days and see if it continues to show these symptoms.

Having said that, I do agree that changing the O2 sensors is a no-brainer just to eliminate that concern. I also found a really great article on the fuel injectors that outlines the exact problem I'm seeing: http://www.zr1specialist.com/HAT%20W...eplacement.pdf

So based on this information I'm still expecting to replace the injectors at some point, if not immediately.

[Paul Workman]

Originally Posted by Reichert

I'm going to drive it for a few more days and see if it continues to show these symptoms.

I'm not sure I'd be driving it if the injectors are going south...

As the 90-92 OE injectors are susceptible to the ethanol in blended fuel, the coils short out (internally) resulting in less fuel being injected: the cylinder burns ever increasingly LEAN and with it all the issues with too lean a mix ensue!

Coincidentally, my motor developed a burnt exhaust valve. If I had kept and inspected the plug from that cylinder, perhaps it would have provided further proof of a lean condition. But, as it happens, the valve was replaced and new injectors (Accel 150821s from Summit Racing) were installed. That was some 6 years and ~ 18k miles ago and NO return of any valve issues...or missing idle...or knock...

SO! Ya might wanna reconsider driving it until you can asses whether or not the car (likely) has OE injectors! Just a thought!

[Reichert]

Originally Posted by Paul Workman

SO! Ya might wanna reconsider driving it until you can asses whether or not the car (likely) has OE injectors! Just a thought!

Thanks Paul. Good advice .. but man it's going to be hard not to drive it at all until I can do the plenum tear down!

Good news is that the gentleman I bought the car from is writing me a $700 check to cover the cost of the injectors. Really generous of him and unexpected. So I can get everything ordered right away, including other things I'll just go ahead and replace while I'm at it (wires, coils, and O2 sensors at least).

As for the injectors .. I had my eye on the FIC set: https://fuelinjectorconnection.com/c...ctor-set-90-92

[jss06c6]

If you're sure the injectors are OEM, you definitely need to change them!

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