530(m60) driveability and idle trouble. Long post need help

[Ross]

Regarding a '94 US 530(m60b30)manual trans 7/93 build
Starts right up when cold but will not idle. Stalls unless the trottle is feathered until completely warm. When warm it idles slow at 500 or less rpm and a bit rough.
All 8 plugs are evenly sooty, no oil fouling. There is some light backfiring in the exhaust if you free rev it to 4k or so. Throttle response is poor. Pipes sooty too. Me thinks it's rich. No codes.
The last time Working on this car was several months ago, my thought was was the ICV had failed, it didn't match the spec in Bentley. A new one ohmed the same as the existing (which has been cleaned) so I wrote it off to Bentley error and returned the new ICV without a trial.
My thinking has been unmetered air entering somewhere causing overcompensation and the richness. No vacuum leaks could be detected.
Since this engine is so given to problems with the PCV I changed it, along with all the intake gaskets and o-rings to no avail. Some oil was leaking from the pipe between the PCV and the oil seperator but no other gaskets looked failed.
Symptoms remain the same or perhaps a bit worse. NO codes.
I'm thinking ICV again but hate guessing with $100 parts.
Weak ignition could cause this, but all 8 coils???? Coils and boots look okay, a little oil in two holes. Again, all plugs are the same.

Thoughts???
Thanks for looking

[Paul in NZ]

o2 sensors?

[bubba966]

The Bentley has the wrong specs for the ICV's as I recall.

Check the resistance on the coils. Easy to do, and it'll tell you which ones are bad, going bad, or are fine. The Bentley does have the proper values for coil resistance. Checked the coils on an M50. 3 were bad, two were right on the verge of being bad, and the last one was getting close to being bad as well.

[Ross]

It gave a code for that once before but I had been fooling around so much creating vacuum leaks, disconnecting the ICV etc. I wrote it of to my tinkering.
Since the intake work and resetting no codes, maybe I ought to drive it a bit more.
I have no code reader and rely on the onboard diagnostic flashes from a "stomp" test.

[632 Regal]

you changed the front plate gasket right?

Backfire through exhaust...rule of thumb, backfire through intake is fuel, through exhaust is ignition.

What about the plug wire boots? TPS loose? Did this happen suddenly or gradually and the intake sealing didnt make any difference?

I would check the things i mentioned but it still sounds like unmetered air to me too.

[Bill R.]

fuel pressure and then check your fuel pressure regulator to see if the diaphragm is torn. May not be rich enough when cold as your describe then too rich when warmed up. Temp sensor can also have an effect here

Regarding a '94 US 530(m60b30)manual trans 7/93 build
Starts right up when cold but will not idle. Stalls unless the trottle is feathered until completely warm. When warm it idles slow at 500 or less rpm and a bit rough.
All 8 plugs are evenly sooty, no oil fouling. There is some light backfiring in the exhaust if you free rev it to 4k or so. Throttle response is poor. Pipes sooty too. Me thinks it's rich. No codes.
The last time Working on this car was several months ago, my thought was was the ICV had failed, it didn't match the spec in Bentley. A new one ohmed the same as the existing (which has been cleaned) so I wrote it off to Bentley error and returned the new ICV without a trial.
My thinking has been unmetered air entering somewhere causing overcompensation and the richness. No vacuum leaks could be detected.
Since this engine is so given to problems with the PCV I changed it, along with all the intake gaskets and o-rings to no avail. Some oil was leaking from the pipe between the PCV and the oil seperator but no other gaskets looked failed.
Symptoms remain the same or perhaps a bit worse. NO codes.
I'm thinking ICV again but hate guessing with $100 parts.
Weak ignition could cause this, but all 8 coils???? Coils and boots look okay, a little oil in two holes. Again, all plugs are the same.

Thoughts???
Thanks for looking

[Ross]

Yes, I changed all the gaskets. And yes I'm beginning to think ignition as well. The first time I posted about this long ago the first reply was "change all the coils",...Great.
I learned the backfire rule with a '72 Riviera that had somehow gotten the distributor clamp loose allowing the timing to radically change one day as I lifted off the throttle from high speed. There must have been lots of unburned fuel in the mufflers 'cause they blowed up real good. I shudder to think what a similar mishap would cost on this car.
I purchased the car this way so have idea if the decline was gradual or not. I suspect gradual as the PO had a fair amount of documentation indicating that if things went wrong they were put right. He stated that he believed the car needed a "tune-up".

[Ross]

Absolutely. I had some notion that the car would overcompensate if the O2 sensors saw the lean exhaust.
I'm going to follow your advice and check those items but am leaning towards ignition faults.

[Tiger]

Don't forget the MAF...

[Ross]

Follow up and conclusion(?)
Intake gaskets were changed along with the PCV, even though no leaks were apparent. NO HELP

All coils, plug "wires" checked okay
ICV cleaned checked, double checked, replaced with two different believed good units, NO HELP

TPS, temp sw, both replaced, NO HELP

MAF exchanged with two seperate known good units, NO HELP

Crank position sensor checked, okay.

Fuel pressure regulator checked and found good, STILL NO CHANGE

FOUND THROTTLE VALVE WAS CLOSING TOO FAR, perhaps throttle stop tab hed become bent or screw was worn down from a million throttle closings. Readjusted and all is well, smooth as silk idle.
Waiting for a cold start now.

What a thrash to find this