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Misfire diagnosis
miguel.760ti Offline
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Post: #1
Misfire diagnosis
So the 760 decided to start misfiring, at first it was only under light load, it idled smooth and ran fine under heavy throttle. However the misfire is now constant and doesn't change with load. I'm starting to think it may be an ignition issue

So far I've checked for vacuum leaks (none found)
Swapped MAFs with no change
Swapped plug wires with no change
Checked O2 sensor volts, they fluctuate normally

I'm gonna be doing cap, rotor & wires this weekend hopefully, but if you guys have any other ideas do share! Also the car is lh 2.2

'88 760ti - Rattley k-car
'89 240dl - Non-running 240 #3 in krisverde's shop
04-11-2012 10:42 AM
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miguel.760ti Offline
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Post: #2
RE: Misfire diagnosis
Just drove the car to work, now it only misfires at idle

'88 760ti - Rattley k-car
'89 240dl - Non-running 240 #3 in krisverde's shop
04-11-2012 11:15 AM
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The Spirit Of Rolling Offline
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Post: #3
RE: Misfire diagnosis
I don't have experience with turbo'ed cars, but could the CTS be faulty?

Fuel Pressure Regulator?
04-11-2012 11:32 AM
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miguel.760ti Offline
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Post: #4
RE: Misfire diagnosis
Its a possibility, but if it was the CTS the car would surge and stall right? The car doesn't stall and doesn't surge much.

FPR only was replaced 20k mi ago, do they fail that often?

'88 760ti - Rattley k-car
'89 240dl - Non-running 240 #3 in krisverde's shop
04-11-2012 12:06 PM
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nathaninwa Offline
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Post: #5
RE: Misfire diagnosis
Do you have a wideband? Vacuum leaks, cracked exhaust manifolds lean the system out, so the ecu then adds fuel to get the stock O2 happy again.
04-11-2012 12:49 PM
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The Spirit Of Rolling Offline
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Post: #6
RE: Misfire diagnosis
I am 90% certain mine is bad. Symptoms, tip-in on the throttle and the car will consider it for a moment, and then go. Worse when cold, still slightly there when warm. It doesn't actually -stall- IMO, it is more of a stutter... There is no surging. I -do- know that since the sensor can be off a few hundred ohms, to having no reading at all, I think the symptoms can vary greatly depending on what it is doing. I learned today (on this board) that the temp sensor overrides any other sensor input too.

I plan to back-probe the wires to the ECU; as I understand it, any increase in resistance of the wires, or even a dirty connection can be disastrous...
04-11-2012 02:39 PM
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miguel.760ti Offline
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Post: #7
RE: Misfire diagnosis
(04-11-2012 12:49 PM)nathaninwa Wrote:  Do you have a wideband? Vacuum leaks, cracked exhaust manifolds lean the system out, so the ecu then adds fuel to get the stock O2 happy again.

I don't, I'll check for vac leaks again and check the manifold.

'88 760ti - Rattley k-car
'89 240dl - Non-running 240 #3 in krisverde's shop
04-11-2012 04:24 PM
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M5Tucker Offline
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Post: #8
RE: Misfire diagnosis
If it's got a dead miss at idle, it shouldn't be hard to figure out what cylinder is misfiring. Pull the plug wires one by one with the car running and see which one doesn't affect engine speed. Go to a parts store and get a spark checker, or use a spare plug and ground it to the valve cover to see if you're actually getting spark to the wire end.

Brickspeed: Yeah? It is? That fast?

-'79 245DL
04-12-2012 06:38 PM
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miguel.760ti Offline
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Post: #9
RE: Misfire diagnosis
Turns out #1 spark plug decided it wanted to have a gap of 0 instead of .030, new plugs/wires/cap/rotor installed and all is well!

'88 760ti - Rattley k-car
'89 240dl - Non-running 240 #3 in krisverde's shop
04-13-2012 12:18 PM
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