HELP! secondary/fuel problem

[george henry]

Hi. New to the forum, so be gentle. I've been reading up on secondary system problems. Full power works fine until the engine gets really hot. Then it starts to fall on its face. Fuel pressure testing shows pressure jump to 52psi, then when the pump shuts off it drops quickly to 42 psi and holds. This is true with the primary pump alone, the secondary pump alone, or both pumps. Engine idle pressue is at 42 psi with the gauge jumping around that number. With engine cold I put a vacuum pump on the line going under the plenum; gauge would very slowly bleed down from 12"hg. With engine hot, no blead down. Car vacuum pump works fine. I have reinforced the air duct so I don't think that is the problem.
HELP WOULD BE VERY APPRECIATED

[tomtom72]

hi George & welcome to the brotherhood!

I'm not too well versed as I'm still a noob....owner since 12/04. But from your description of events I'd say the pumps check out okay except for the "dancing" part....when was the F/F last changed? It could be loaded up & have a partial blockage? Easy & cheap to swap out.

There is a method to check the ohms on the injectors thru the ECM harness or thru the secondary injector relays (I think?). The injectors could be having a coil failure at elevated temps. I believe anything above 18 - 20 ohms is a good number, less is fubared. If you have a recording scan tool you could hook it up & drive around & record the fuel numbers and O2 readings for later analysis....that would tell you if the fuel flow is proper from the injectors. If the car runs well at idle in closed loop then I think the 1* injectors are not the issue but that's worth scanning also. If you have a 90 or a 91 you have the old style injectors that are not ethanol proof and the coils will fail eventually due to the ethanol but usually it's the primaries that fail first. You don't have any DTC's in the ECM or the CCM? I would also suspect the spark plugs and wires and maybe the ing coils as you say the heat has to be up before symptoms start. Usually if any of that stuff is marginal, the hotter it gets the worse it performs. Heat will cause electrical break down.

I also don't get what's up with the vacuum check: cold you have a leak down and hot you are solid....I believe the FSM has a leak down rate on the secondary vacuum control system. Still that seems counter intuitive as a test result. How long does it take to leak down to zero? If longer than a couple of minutes I'd say that the system passes muster.

Some of the more senior guys will see this and give you better pointers before too long. Hang in there!


Tom

[tccrab]

Originally Posted by george henry

Hi. New to the forum, so be gentle. I've been reading up on secondary system problems. Full power works fine until the engine gets really hot. Then it starts to fall on its face. Fuel pressure testing shows pressure jump to 52psi, then when the pump shuts off it drops quickly to 42 psi and holds. This is true with the primary pump alone, the secondary pump alone, or both pumps. Engine idle pressue is at 42 psi with the gauge jumping around that number. With engine cold I put a vacuum pump on the line going under the plenum; gauge would very slowly bleed down from 12"hg. With engine hot, no blead down. Car vacuum pump works fine. I have reinforced the air duct so I don't think that is the problem.
HELP WOULD BE VERY APPRECIATED

George:
It doesn't sound like fuel injectors, but's here a link to a way of testing them without taking the plenum off of the car:
http://www.zr1specialist.com/HAT%20W...Resistance.htm
I'd be more suspicious of ignition related items, coil packs, spark plug wires or spark plugs.

Just my $0.02

TomC
'90ZR1 #792

[george henry]

FYI: The fuel filter, plugs and wires are new.

[tomtom72]

Originally Posted by george henry

FYI: The fuel filter, plugs and wires are new.

Okay, I'll take another try. You seem to have eliminated alot with replacement parts. If you have use of a scan tool I would use it to see what's happening when the motor gets to that temp where it gives you trouble. That might be easier than Ohming the 2* injectors??? Only reason I say scan it is that there are other sensors that are important to 2*operation like the O2 sensors and a scan tool will tell you what they are telling the ECM, and you also get the integrator and block learn and cell #'s to boot. Oh, and also you get to see what the real engine coolant & oil temps look like to the ECM.

My line of thinking is this: the ECM has built in safe guards that cancel 2* operation if a DTC is stored in memory or the temps get too far out of line...either high or low. Also, the scan will show all the feed back inputs that the ECM is seeing, which could show what is causing the problem when it is actually happening. JMHO.

The only other thing I can think of, with respect to heat, is the brain box for the ignition, that must get plenty hot where it lives. That part I've no idea how to test.


Tom

[george henry]

A week at "The Corvette Shop" in San Diego and $1200 showed the problem to be a bad coil pack. Replaced all 4 coils. She seems fine now.

[guinnessdood]

Originally Posted by george henry

A week at "The Corvette Shop" in San Diego and $1200 showed the problem to be a bad coil pack. Replaced all 4 coils. She seems fine now.

Wow...$1200! Next time buy the manuals and do it yourself...it's not that big of a deal to pull the plenum. About 3 hours and you would have been done....and for the price of 4 coil packs (~$40 each). you can usually even reuse the gaskets. If you need help with it...just ask us. :-) We are here to help!