Advanced engine tune troubleshooting thread
Camel Trophy
Joined: Nov 2012
Posts: 3,717
Likes: 248
From: Longview, Texas
It could also be the MAF. I was having crazy problems exactly as you describe and my rover was gimped for WEEKS while I changed this and that. DiscoMike said swap the MAF and when I did it all went away. I'd changed so many things it could have been a combination.
I was lucky enough to hav a buddy who has the same rover and let me borrow his MAF.
I was lucky enough to hav a buddy who has the same rover and let me borrow his MAF.
Last edited by Charlie_V; Jul 13, 2015 at 01:00 AM.
Never thought of the MAF, I wonder why it would miss on Cylinder #7 and only on startup with a bad MAF?
disc oh no - plug wires are new - I changed the new ones that were on the truck to new ones I had already just to double check, no change.
Hmmm, could be that dreaded end cylinder head gasket leak - leaking cold, not leaking hot and expanded - most likely answer I have heard so far (unfortunately). I wonder if a pressure test would find it?
RE: ECM rich lean cycle - I'm sure you already know this but for the benefit of our other readers - the ECM treats the O2 like a hi/lo switch - increases fueling of the injectors till it sees high, then decreases till it sees low - alternates between the two ditches so that average is in the middle - you can see it if you have a scanner with o2 voltage readout. BTW, almost all ECM's work that way - at least most of the one I have worked on/tuned which has been several.
disc oh no - plug wires are new - I changed the new ones that were on the truck to new ones I had already just to double check, no change.
Hmmm, could be that dreaded end cylinder head gasket leak - leaking cold, not leaking hot and expanded - most likely answer I have heard so far (unfortunately). I wonder if a pressure test would find it?
RE: ECM rich lean cycle - I'm sure you already know this but for the benefit of our other readers - the ECM treats the O2 like a hi/lo switch - increases fueling of the injectors till it sees high, then decreases till it sees low - alternates between the two ditches so that average is in the middle - you can see it if you have a scanner with o2 voltage readout. BTW, almost all ECM's work that way - at least most of the one I have worked on/tuned which has been several.
Recovery Vehicle
Joined: Nov 2010
Posts: 948
Likes: 36
From: New Hampshire
Yes, the ECM takes the input from the o2 sensors and uses the readings to determine fuel injector pulse width. I was just saying that the millivolt readings that you see are produced by the sensors themselves.
As far as a gasket leaking into a cylinder, I've seen head and intake manifold gaskets do the same thing. I'm not sure if they actually stop leaking when the engine warms up??? They might, or if the leak is very small, it may take hours to build up enough coolant to cause a miss. So it could be leaking all the time, but maybe it's only noticeable during first start up.
I've had a customer car where it leaked enough overnight to noticeably slow the starter when I started it to bring it into the shop. That's scary! Fortunately, the crank didn't get damaged.
As far as a gasket leaking into a cylinder, I've seen head and intake manifold gaskets do the same thing. I'm not sure if they actually stop leaking when the engine warms up??? They might, or if the leak is very small, it may take hours to build up enough coolant to cause a miss. So it could be leaking all the time, but maybe it's only noticeable during first start up.
I've had a customer car where it leaked enough overnight to noticeably slow the starter when I started it to bring it into the shop. That's scary! Fortunately, the crank didn't get damaged.
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