M42 strange hesitation problem

[318Eric]

SO, let me begin by saying I've already breezed through the "official hesitation/bogging" thread, as well as searched for other 318 hesitation threads, but the "official" thread seems to pertain almost exclusively to M50/52's, and the 318 threads were mostly left open-ended.

I've had the car down for a few days, fixing a few problems that have been on my list forever. My main goal was to replace the front main seal and the oil filter housing gaskets. Unfortunately, I broke my alternator in the process so I had to replace it w/ a junkyard part.

The car started, idled, and ran perfectly last night when I got it back together; even ran it up to 90MPH for sh!ts and giggles on an open stretch of freeway-no problems. This morning, however, was a different story. It cranked up and idled perfectly fine. But once I hit the road, I discovered it wouldn't rev past 3,500 RPM; it was as though it was hitting the rev limiter, coughing and misfiring and just refusing to rev any higher, but would run smooth as silk at anything below, w/ good throttle response, etc. As the car warmed up, the problem gradually went away, 1k RPM at a time, till it would run all the way to redline if I tried once it was up to temp.

My first thought, obviously, is to double-check my handiwork, and I'll do so this evening. But in the meantime, I'm trying to decide what I think the problem might be...

I removed/disconnected/f'ed with in some way the following:

-battery
-alternator (replaced w/ an identical unit from an identical car which I'm told by a friend had 100k fewer miles and ran before it went to the yard)
-oil pressure sending switch
-MAF, upper airbox, intake elbow & assoc. vacuum connections
-unbolted and pushed the evap canister back in the engine bay to gain access to alternator connections; also removed but re-installed a line off the front of this

I guess it's worth mentioning that it was down to ~34* this morning, the coldest day of the season so far here. No CEL, etc.

Thoughts?

[por911]

Check your fuel pressures(running and rest). Replacing the t-stat , coolant temp sensor, and O2 sensor really helped the performance and mpg on our 318ia.

You might also check your PCV(black circular part on intake in pic).
regards

[318Eric]

Have recently (a yr. or so ago) replaced the t-stat, and even more recently had an injector fail (which acted like a dead miss); figured an O2 sensor might be my problem, since my first thought is it's running better once in closed-loop.

Quick (not necessarily substantial) update. I've been in over my head with work lately, so didn't get a chance to look over things last night. However, I can say that the problem is consistent: cold start= 3500 RPM max, which gradually increases with engine temperature, till it runs perfectly normal once the temp. guage reads just below half.


I'm leaning towards this being something electronic and "deliberate" from the standpoint of the ECM, because when it begins to cut out, it is a very "precise", "rhythmic" thing; in other words, it doesn't feel to me like it is a lack of fuel pressure, as some had suggested, cause I would imagine if that were the case, the engine would feel sluggish or wouldn't cut out so abruptly. It feels to me like the ECM is cutting either spark or fuel because it runs extremely strong, right up to wherever it falls off. And if it were a fuel problem, why would it gradually go away as the car warmed up?


Obviously, looking at it will tell me more; I just haven't had a chance to and am continuing to mull over it in preparation of actually getting my hands dirty.


I was wondering, however- do these cars have some sort of cut-off related to oil pressure or coolant temperature? (in other words, can the ECM cut fuel/spark if it senses oil pressure is too high/low, or can it limit RPM's if the coolant temperature isn't high enough?)