Engine surge or intermittent power loss at part throttle

kmead

kmead

Old enough to know better
My 1985 X has a surge or rhythmic loss of power when hot between 3500 and 3900 rpm, it is noticeable but not debilitating.

I haven't found a reason for it but wonder if it is a fuel pressure issue, a vacuum issue that is affecting the distributor advance mechanism or ?

On my car it is a slight bucking or surging/loss of power with light to medium throttle openings and goes away if I drop a gear to go beyond the rpm range, bury the throttle to move the car through the range or upshift to drop below the range. Odd. It will happen in all gears so it is engine and rpm related ruling out engine mounts, CVs or other rotating mass issues.

I don't own a fuel pressure gauge so I can't immediately use that to rule out pressure and as it is at speed and under load it likely wouldn't be immediately obvious.

Vac lines appear good though I am going to go on a trip to NAPA to grab some replacement hose just to be sure its not a vacuum advance induced problem.

Thanks for in advance of any suggestions.
 
If that is the range that you typically operate in, then it may well be carbon tracking on the AFM sweep, so more to do with injector flow/volume.

It is possible it is fuel pressure, but that would seem unlikely if it is limited to such a specific rpm range.

If you have no luck with the ignition / vacuum side of things, might be worth swapping in another AFM, to try, assuming you have a spare...
 
Thanks that is an interesting angle on the problem. I hadn't thought of that, I will pop the top of the existing one I have and see if there is any evidence of carbon or other wear in the AFM. I have never acquired a back up AFM. I could swap the one from the other X to test.

Great suggestion, thank you.
 
Test the O2 sensor with a voltmeter.. I replaced mine and the overall smoothness of the engine improved dramatically....
 
kmead said:
My 1985 X has a surge or rhythmic loss of power when hot between 3500 and 3900 rpm, it is noticeable but not debilitating.

I haven't found a reason for it but wonder if it is a fuel pressure issue, a vacuum issue that is affecting the distributor advance mechanism or ?

On my car it is a slight bucking or surging/loss of power with light to medium throttle openings and goes away if I drop a gear to go beyond the rpm range, bury the throttle to move the car through the range or upshift to drop below the range. Odd. It will happen in all gears so it is engine and rpm related ruling out engine mounts, CVs or other rotating mass issues.

I don't own a fuel pressure gauge so I can't immediately use that to rule out pressure and as it is at speed and under load it likely wouldn't be immediately obvious.

Vac lines appear good though I am going to go on a trip to NAPA to grab some replacement hose just to be sure its not a vacuum advance induced problem.

Thanks for in advance of any suggestions.
Click to expand...

Karl,

Pretty sure you know this but ALL the major auto parts chains will be happy to loan you a full fuel system test kit as part of their "tool loaner" program. I just checked the AutoZone site and they have a very nice one to loan with lots of fun toys! http://www.autozone.com/loan-a-tools/fuel-pump-diagnostic-kit

Ed
 
Actually I was not aware, I tend to stay away from most auto parts stores as they generally are not helpful

I will look into borrowing that setup as they are just up the road from where I work.

Thanks!
 
ecohen2 said:
Good point!! Does that same sensor feed the temp gauge?
Click to expand...

No. The FI Coolant Temp Sensor is located in the stat housing and has your typical Bosch 2-wire plug on it. The sending unit for the instrument panel gauge is located on the spark plug side of the head between #2 and #3 and has one (IIRC) green w/white tracer wire going to it, with a brown rubber boot. On FI cars, the temp gauge sender has the Thermo-Time Switch as a next-door neighbor.
 
So far so good, I cleared the vacuum port at the base of the throttle plate and it seems to have resolved my problem. It will be hot this week so I will be better able to test the efficacy of the 'fix'. I really need to pull the intake off and clean it. Lots of oily grunge in there which I am sure is not adding to the drivability of the car.
 
You must log in or register to reply here.
Back
Top