Knowledgeable Tech -Starting/Performance problem needing assistance

[sjs]

Hello everyone, I am new so please bear with me. I am new to the forum and this is the first zr-1 I have owned (90 model w/100k). Bought it last year, runs pretty good but secondary actuator does leak. I am an engineer but hate to admit I haven't bought a service manual yet (toys never break do they? LOL) Anyway, drove it for about 3k miles last year, wife drove it the first day of spring and as she turned in the drive it ran rough and died. Would start and run, but sounded and smelled flooded - would start after it dried out ... but would die. Put it in clear flood mode and will start but runs tough for a few seconds and dies. It used to start wonderful and run hard .. now it is becoming an annoyance and a reluctant wife is starting to loose faith and trust in her first vette. So if anyone has any ideas I would love to hear from the people who know the most about them. When it does run it pulls 11in of very steady vacuum at a low/rough idle , I thought it sounded throaty but funny (faint popping through the intake (had filter out), and tested fuel pressure at 39 psi (it does hold pressure too). So I am at a loss for what its malfunction is as I am NOT heavily familiar with this engine - not near as well as an l98 i used to have in a trans am. Could this exotic 4 cam engine have a jumped cam as they are known to have tensioner problems? But why does it have steady and almost good idle vacuum? Bad injectors ? But why will it run good for a few seconds before going down and why do the hold pressure (ie not leak), Scanner showed only lean code and secondary actuator code (i assume from its slow secondary vacuum leak) ... So I assume spark and ecm are okay since it kinda runs ....

Anyway, LT5 experts ..... please help and thanks in advance.
sjs

[Z51JEFF]

Since the injectors are the first thing to fail I would look at those first.No trouble codes,check engine light?Regardless the problem is not a major one,these motors are not known for breaking.

[A26B]

While I do agree completely with Z51Jeff, regarding the need to ultimately replace original injectors, especially for 90~92 models, I would also suggest the following posibility that can be easily checked & remedied if necessary.

One of the more common ocurrences is a MAP sensor hose that has come loose. The LT5 with its waste spark system, requires that during starting, the key is held in the start position momentarily after the engine starts. Otherwise, on ocassion, the engine will backfire and the suden manifold pressure will blow the connector hose (plenum to MAP sensor) off.

Here is a photo showing the placement of the MAP Sensor hose.

[gbrtng]

When my 91 sneezes and blows off the MAP hose it sets an SES code and light.

sjs: does it set any SES codes?

[tccrab]

At 100k miles I'm willing to be that it's already had the injectors replaced.
That being said, it could still be bad injectors, especially if the previous owner had them replaced with more of the Multecs.
Check the MAP hose first as Jerry said above.
Maybe one of the Brotherhood local to you wouldn't mind meeting with you for a semi-educated diagnosis.


TomC
'Crabs

[Jagdpanzer]

Where abouts in Kansas are you located?
I grew up in Garden City, KS and now live in Northern Virginia.

[sjs]

Originally Posted by Jagdpanzer

Where abouts in Kansas are you located?
I grew up in Garden City, KS and now live in Northern Virginia.

We are up in the northeast corner by Manhattan KS, GO KSU!

[Paul Workman]

Originally Posted by tccrab

At 100k miles I'm willing to be that it's already had the injectors replaced.
That being said, it could still be bad injectors, especially if the previous owner had them replaced with more of the Multecs.
Check the MAP hose first as Jerry said above.
Maybe one of the Brotherhood local to you wouldn't mind meeting with you for a semi-educated diagnosis.


TomC
'Crabs

BINGO!

That is what happened on mine. The injectors were replaced with more of the same (GM dealership before I bought it). They crapped again as well. The new Accels made and incredible difference in overall operation, and especially at idle!

And too, as pointed out, the fuel pressure situation has to be resolved first. That issue alone can be the root of a lot of evil.

To the OP: the O2s are not going to come into play until the thing warms up some, but once you get to that point where it goes into "closed loop" mode, you might want to compare integrator values and O2 cross-points. First thing tho...is that fuel pressure issue!

P.

[lbszr]

Was the fuel pressure monitored while it was trying to run, or with the key on? It might be falling even lower than 39 while it's running.

[sjs]

Originally Posted by Paul Workman

BINGO!

That is what happened on mine. The injectors were replaced with more of the same (GM dealership before I bought it). They crapped again as well. The new Accels made and incredible difference in overall operation, and especially at idle!

And too, as pointed out, the fuel pressure situation has to be resolved first. That issue alone can be the root of a lot of evil.

To the OP: the O2s are not going to come into play until the thing warms up some, but once you get to that point where it goes into "closed loop" mode, you might want to compare integrator values and O2 cross-points. First thing tho...is that fuel pressure issue!

P.

Thanks, am working on the fuel psi right now ... will let everyone know