1991 zr1

[kengodfrey]

I have 1991 zr1. It has 19000 miles. It has developed a knocking noise after ir runs about 15 minutes. Mechanic said it was running on 7 cylinders. Could be plug wires or injectors. Any comments are suggestions?

[tomtom72]

Hi Ken, If the original GM injectors are still installed then it's a good place to start looking. The OEM injectors are not resistant to the ethanol used as an additive in our gas now a days. Generally when the OE injectors start to go bad it takes a little time for the coils to start shorting out. Usually about the time the coolant temp gets to closed loop the miss starts.

Did you try to take an ohm reading on the injector coils?
Did your mechanic pull the plugs to eye them up?
Did he take an ohm reading on the plug wires, from one plug thru it's assigned ign coil thru to it's other plug in the paring?
The knock is the dual mass flywheel responding to uneven ( a big miss ) power pulses.

Does the car smell lean, or rich at the exhaust outlets?
Did he put a scan tool on her at idle to look at the fuel numbers, cross counts, O2 voltages?



Tom

[kengodfrey]

Wow, you nailed all the symptoms! The car runs perfect until it warms up, and thats when the knock starts. The flywheel makes sense; when I push in the clutch, the noise diminishes. I didnt have time to leave the car, but will take it back. Thanks for your response; could not understand why it ran perfect for 15 minutes and then problem started. Thanks again, Ken.

Originally Posted by tomtom72

Hi Ken, If the original GM injectors are still installed then it's a good place to start looking. The OEM injectors are not resistant to the ethanol used as an additive in our gas now a days. Generally when the OE injectors start to go bad it takes a little time for the coils to start shorting out. Usually about the time the coolant temp gets to closed loop the miss starts.

Did you try to take an ohm reading on the injector coils?
Did your mechanic pull the plugs to eye them up?
Did he take an ohm reading on the plug wires, from one plug thru it's assigned ign coil thru to it's other plug in the paring?
The knock is the dual mass flywheel responding to uneven ( a big miss ) power pulses.

Does the car smell lean, or rich at the exhaust outlets?
Did he put a scan tool on her at idle to look at the fuel numbers, cross counts, O2 voltages?



Tom

[Kevin]

i'm willing to bet it's injectors

[ScottZ95ZR1]

Exactly my symptoms with the '91 I bought last year with 17,000 miles.

Ran fine on short test drive but the first time I got it good and warmed up - sounded like the flywheel was going to come through the hood.

Marc's article describing the injector coil failure and testing procedures from the ECM connectors helped me confirm two bad primary injectors.

Check it out: http://www.zr1specialist.com/HAT%20W...in%20Place.pdf

Recommend this vendor for replacement injectors: http://www.corvetteinjectors.com/sho...p?productid=32

[FU]

Originally Posted by ScottZ95ZR1

Recommend this vendor for replacement injectors: http://www.corvetteinjectors.com/sho...p?productid=32

That's the ticket Scott

[Blue Flame Restorations]

Originally Posted by ScottZ95ZR1

Exactly my symptoms with the '91 I bought last year with 17,000 miles.

Ran fine on short test drive but the first time I got it good and warmed up - sounded like the flywheel was going to come through the hood.

Marc's article describing the injector coil failure and testing procedures from the ECM connectors helped me confirm two bad primary injectors.

Check it out: http://www.zr1specialist.com/HAT%20W...in%20Place.pdf

Recommend this vendor for replacement injectors: http://www.corvetteinjectors.com/sho...p?productid=32

Wow. Did the previous owner hint of the issue when you looked at it?

[tomtom72]

Originally Posted by kengodfrey

Wow, you nailed all the symptoms! The car runs perfect until it warms up, and thats when the knock starts. The flywheel makes sense; when I push in the clutch, the noise diminishes. I didnt have time to leave the car, but will take it back. Thanks for your response; could not understand why it ran perfect for 15 minutes and then problem started. Thanks again, Ken.

hey man that's what we do here....we help each other keep these cars running right!

Look, this will get worse over time. It may get to the point that the miss will start almost upon start up. It's the epoxy on the coil windings that is the issue. The more exposure after the alcohol gets to eat thru some of it, the faster that hole becomes bigger.

A scan in closed loop at idle will yield the BLM's & INT's, and O2 info...they will tip you to the ECM trying to add fuel to make up for the injector(s) not squirting fuel. Also, the p[lugs will be a tell tale as the holes with the bad injectors will look too clean to have had combustion. Also, if the miss is really bad at idle, go sniff the exhaust. If it make your eyes water and smells pungent then that's you smelling a lean condition.

Also, when you do an ohm test on the injectors you can go thru the ECM harness. A word of caution here. The ECM plugs have very small female terminals, so don't stick a normal test lead from a DVOM in them, use some phone wire. Reason is that you can deform the terminals and end up with an intermittent contact! Be very careful if you ohm the injectors that way.


Tom

[mike100]

Replace the injectors no matter what- they all fail. Even if you still have another problem, its still a service item that needs to be handled.

and as far as the 'previous owner didn't mention it' comment...haha of course not- mine didn't either.