Wouldn't hurt checking. It does start to sound more and more like an intermittent sensor issue. The trouble is finding which one? I guess you start the engine, let it warm up and start yanking on wire harnesses and connectors until engine starts hiccuping.

 

Yea pretty much. Or with something like the handle of a screwdriver tap on sensors.

 

Ricardohd91: I looked up that "AFM"... could only find "MAF": Mass Airflow Meter. Is that the same thing?
OK... I'll unplug that before I drive in the morning, and see what happens.
IF cleaning that solves the problem, then... you're not a dunce... You're a friggin' Genius !

Today, however, guys, I did the Fast acceleration test... 8 or 10 times.
MILDly accelerating, at 30 to 45 MPH, it started missing.
I immediately floored it... and the missing Stopped.
I'm thinking... probably not a fuel supply problem, because it got lots of fuel when floored. Yes?
I think that means that it IS an Open / Closed loop problem.
So... what does That mean? Are the O2 sensors bad? Something else?

Thanks, again.

 

Sorry... I didn't notice that this had moved to page 2.
OK, Mr. Hunter, it could be an intermittent Sensor problem? But... the problem is not noticable at idle.
The problem appears only when Mildly accelerating, between 30 and 50 MPH. Really... it's only noticable Then !
It seems to idle well, and it cruises at 65 MPH without a noticable miss.
WHAT could cause an Intermittent problem ONLY in that narrow speed range?
45 years ago, my brother and I used to adjust the timing on our VW Bugs by turning the distributor while standing on the back bumper
and driving down the road, but I really don't want to "Hit" various sensors on this Mitsu while at 35 MPH.
I'm Way Too old fer that crap.

 

MAF is the same thing. Do you know of any shop in the area that has a dyno? If you had access to a dynamometer you could "drive" it how you know it misses and see if it leans out or goes rich.

 

Hey Mark53, good investigative work so far. I agree that you most likely eliminated the fuel supply issue. You are correct - low fuel pressure or some other issue with fuel would show up more readily with higher fuel demand (wide open). I'd say that the main suspects now are sensor or wiring issue.
To test the MAF you do not have to "hit" it hard. Just tap on it with the back of a screwdriver. Also, wiggle the connector and all individual wires going to the MAF. If you disconnect MAF completely, the engine will not run, so that's not going to work.
You can try one more test, or perhaps you can answer this already - You said engine misses consistently at certain speed range. Does this mean it will miss at all gear ranges at specific RPM range or ONLY between 30 and 50 MPH? If you can make it miss at certain RPM range, the shop (or you working in the garage) should be able to duplicate this with vehicle in Park while revving up the engine. In this case you can rev up the engine to the suspect RPM range, hold it there and check the sensors by pulling and moving the wiring harnesses. I'd start by checking and cleaning all ground connections on the engine. It is possible that a miss happens when a bad connector or bad ground resonate with engine vibrations at certain speed. Also, check the engine mounts. If they are worn or broken, the engine may move/shift enough in the engine compartment to put a strain on a part of the wire harness, thus causing the misfire.

As far as PCM switching back and forth between Open and Closed loop with maximum throttle position, this may have been the case in older designs. New/current vehicles will stay in Open Loop after warm up, unless a critical sensor stops reporting. Then PCM will switch to the "backup" mode which is Closed Loop. If this were the case you would have the Check Engine Light illuminated (would be helpful...).

Good puzzle you got here

 

Try one more thing - park the vehicle with engine running. Gradually increase RPM through the entire range. Do this gradually up and down several time. Note any possible irregularities in how engine runs. If you do notice any irregularities at a specific repeatable spot - you may have a bad Throttle Position sensor. This would explain why engine runs well at idle and at full throttle but stumbles in mid range. You can also test the TPS with engine off if you connect and Ohmmeter to the signal wire on TPS and move throttle through the range. This may be more accurate way to test it.

 

^^^^ What he said.

 

Well, gents, I unplugged the MAF, and drove it for 5 miles... ran about the same, but the miss was Also there at Idle.
So, I plugged it back in. This IS the MAF,... yes?...

I'm not sure about if the miss is at a certain MPH or RPM. I'll try to determine that.

Testing the TPS sounds like a great idea. I might get to that tomorrow.

Question... this thing IS still reliable, and safe,... yes? I'm planning an 8-hour drive to another city tonight, about 500 miles.
And back in a few days. THIS problem won't leave us stranded somewhere, will it?

Thanks very much for your continued interest.

 

I don't know why that picture is not appearing.
https://www.dropbox.com/s/yf11krxfk5...20500.jpg?dl=0

https://www.dropbox.com/s/yf11krxfk5...20500.jpg?dl=0