Stumbling question....

[DanS HJ-45]

OK, 1985 4-runner. Stock.

It occasionally has fits of stumbling. Wierd thing is, never a CEL. I drove it from Barstow, CA home to Colorado yesterday, and what's odd is that it seems to get better the longer it drives. But stop for an hour for lunch? Back to stumbling.

Less than 3000 miles since the new fuel filter, maybe 5000 since I replaced the timing chain and did a lot of PM associated with that.

Here's the odd part: it usually gets a fit of stumbling after hitting a bump in the road, but not always. It also seems worse when the throttle is fully depressed but the engine is lugging at low RPMs, but even that is kind of hard to pin down.

I'm off to town to go give the TPS and MAF a good cleaning, but I'm suspecting fuel contamination. The truck overall is kind of rusty and whatnot, plus it generally lives in an airport parking lot and doesn't get driven a whole lot. I'm wondering if it got some water in the gas tank (it sat for nearly 6 months a while back), and then some rust in the tank is clogging stuff up, but only after a bump knocks the crud loose or something? I'll probably drain the gas too, just to check, while I'm there...

Any other ideas? I didn't see any grounds that looked bad, and the stumbles are SHORT, fractions of seconds kind of duration. It has never died from them, and it generally idles just fine.

EDIT--a minute after posting this WristPin called me back and his suggestion was to clean the Throttle Body, and disconnect the EGR vacuum line to rule that out. So at least I might be on the right track....

Dan

 

[Bogo]

The '95 3.slow 4runner I just got also seams to be doing similar. Haven't had the time to trace down any threads on what may be at issue. I think colder temperatures makes the stumbling worse which makes me suspect the engine temperature sensor or coolant. The other thing I'm suspecting is the AFM. It may not be moving immediately when the airflow into the engine changes. It may need some road vibrations to become unstuck so it can move to the right position. Colder temperatures could make the flapper in the AFM less likely to move when it needs to.

 

[DanS HJ-45]

Quick update: a few bottles of HEET in the gas tank and I cleaned out a TON of gunk from the throttle body.

No change.

Tomorrow I'm going to recheck the timing, just to be sure, but I'm planning on draining the gas tank to check for particulates, instead of just water. It DEFINITELY acts up for a split second after hitting bumps, so I wonder if the sediment it getting stirred up or something...

Dan

 

[DanS HJ-45]

Quick update:

I pulled and cleaned the Mass Airflow Sensor, I drained the gas that was in the tank, and I also retarded the timing just a touch (not all at once).

Definitely better, almost fixed. In 100 miles, it maybe has a hiccup once or twice. I'll probably retard the timing just a touch more, to see if it's somehow better that way. Problem with the timing on this truck is, that it's the one nobody can find the engine test plug on (long story), so I can get it warmed up and stuff, but it's hard to estimate just where it is supposed to be.....

Dan

 

[pappy]

Sounds like the problem I'm having Dan. Sure would like to figure this one out. Also, I've been talking with Ryan, who is also stumped at this time. I'll send you what he suggested.

 

[DanS HJ-45]

Got your message. I'll definitely take a look at the wiring harness and crank pulley....

Dan

 

[Bogo]

So far I've run a couple tanks of gas with injector cleaner and it's better, but not perfect. I suspect I will still need to go in and do a full tuneup as it has never been done on this truck.