Diagnosis needed TIA

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demarpaint

demarpaint

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I've posted a few times about a surge and stumble with my 4.9L 88 E- 150 5 speed stick van. Long story short every tech that looked at it threw his arms up and surrendered. It is more of an annoyance than anything else and has been going on for a very, very long time.

While waiting for my wife today I was idling the van, and noticed on the tach every time I blip the gas, the rpms would drop about 200 rpms then react to the blip of the gas. This is exactly what happens when taking off from a dead stop engaging the clutch and feeding her a small amount of gas. It will stumble exactly the way it does when I blip the gas idling. Everything as far as sensors, vacuum leaks, voltage readings etc have been checked, and verified OK. I'm interested in hearing what you guys think, after this most recent discovery.

TIA
Frank
 
If carbed- accel pump as mentioned above or carb body loose from throttle plate. Loose carbs are epidemic on those.

FI- If MAF equipped then a MAF problem. If MAP equipped, ck for clogged/collapsed hose between MAP and manifold, ck for "jelly" in the vac connection/hose at the MAP. Use a DVOM and do a sweep test on the TP sensor. Screwed up injector spray patterns. Injectors are pre DRI and are prone to clogging/messed up spray patterns.

Generally a "tip in" hesitation/stall is due to a lean condition so that is what you look for first. If it was an ignition (coil) problem it would evidence itself under heavy load as well, not just tip in.
 
Originally Posted By: daman
Is it a miss stumble or smooth accelerate stumble? if you get what i'm trying to say..


4.9 L I-6 EFI.


If I blip or feather the gas, the rpm drop ~200 there is a very slight pause and then the rpm increase to a point proportionate to where the gas pedal was. So it could pop to 800 rpm or higher. A slight annoying delay/stumble for a stick driver, I guess that is the best way to describe it.

A good Bitog buddy said it could be the thermactor air pump which is something that really hasn't been checked, yet! The pump is a bit noisy, and has been for years, I was told to leave it alone, so I did.

Here's a small list of what's been checked. Many of these parts were borrowed from another vehicle same year w/o issues, to test and help r/o possibilities.


No vacuum leaks confirmed twice with smoke test, and two more times by me with carb cleaner, and propane.

IAC is new and clean, TPS is new, new gas filter, FP pressure regulator, O2 sensor, MAP sensor replaced line was perfect, CTS, Air Charge Sensor, we even tried a computer, checked grounds and voltage readings of sensors, which I tossed when we confirmed they were in spec. EGR was replaced, and is now blocked with a plate I made to test if that was the problem. I did gain some power blocking the EGR valve. I also tossed on an updated Throttle Body Gasket and made sure that was clean as well. Much to my surprise the IAC was very clean along with the area it mates to the engine. Once this thing is moving it runs perfectly and has a ton of power.

Injectors cleaned twice via the rail, I was told the injectors were fine.
 
My Bitog buddy and I are pretty sure it is a lean condition and think the air pump could be causing it. He is looking into that for me, and then we'll try to figure out a way to bi-pass it to check.

Pulling the injectors on a hunch is a big job, 3.5 hrs according to all data, and as mentioned the techs that had it were fairly certain they were not the problem.
 
In cases like this I find it best to give up, have a beer, and look for other challenges.

Pull the belt off the thermactor pump and see if there is a difference. It might possibly be effecting the O2 sensor due to a sticky diverter/dump valve. Slim but possible. EEC IV tests the diverter valve/thermactor pump every drive cycle.
 
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Ok i was just wondering i'm not a ford guy but once on my z71 i had the same issue a pause stumble upon a stop sign start it turned out to be a bad plug wire grounding out.

has plug wire cap/coil packs,etc been looked at?
 
I've tossed in the towel more than once and had quite a few beers over this thing. In 2009 I left it with a guy for a little over a month and he tried everything. He is a well respected tech, a good guy, he told me to live with it. He charged me for an hours labor part of which was the FI cleaning and him telling me they were good. In 2010 I left it with another tech friend he and his boss went through it again, said it was a lean condition and had no fix. They all ran lean IIRC. No one mentioned the thermactor pump until today.

The stumble is very annoying because it is a stick, an AT would be a bit more forgiving. I've lived with it this long I guess I will continue to.

Its a shame, the body and paint are in great condition as well as the interior. It turns heads when its all cleaned up.

Edit to add:

It was tuned up less than 5000 miles ago. Cap, wires, rotor, ignition module are all good, timing is spot on.
 
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Originally Posted By: demarpaint

Edit to add:

It was tuned up less than 5000 miles ago. Cap, wires, rotor, ignition module are all good, timing is spot on.

Well it could be a new issue since them 5,000 miles,mine had me stumped because they were new Beldins from NAPA 1 year ago so i kept over looking that area as a possible problem.
 
I remember the last time we discussed this, lol!

Did you perform the base-idle reset procedure?

Just curious.... If you hold the pedal so the idle is JUST above "idle", and let it sit there. Then perform your "tip-in" from that point, does it still do it?

Curious as to whether it is the transition from idle to throttle that is causing it, or if any sharp blip in throttle actuation from a low RPM point causes the momentary lean problem.
 
Originally Posted By: OVERK1LL
I remember the last time we discussed this, lol!

Did you perform the base-idle reset procedure?



You have a good memory, you were a big help then, and thanks again for taking a stab now! The base idle was properly set and reconfirmed. Interesting note, a tech had messed with the base idle and set it way to high, I still had the stumble, and surging on a hot restart. That is another issue that I wrote about once before which is tied into all this mess. Here I wanted to address one issue at a time but I'm certain both problems are caused by the same thing.


Originally Posted By: OVERK1LL
Just curious.... If you hold the pedal so the idle is JUST above "idle", and let it sit there. Then perform your "tip-in" from that point, does it still do it?


Good question I'll have to try that.


Originally Posted By: OVERK1LL
Curious as to whether it is the transition from idle to throttle that is causing it, or if any sharp blip in throttle actuation from a low RPM point causes the momentary lean problem.


The blip is literally a feathers touch, a slight tap that in any normal properly running engine should bring the idle up maybe 100 rpm, in this case it drops ~200 rpm then pops up after a slight half second delay. Once in motion there are no issues, it is only moving this thing from a dead stop. If I give extra throttle taking off the stumble will be more pronounced. I would say this occurs 6 or 7 out of 10 times taking off from a stop, or blipping the gas when it is idling.

Daman- This problem has been with this van for a very long time, before and after a tune up, I'm positive it has nothing to do with the tune up parts, but thanks for taking the time to help!!!!!!!!!!!
 
What if you force it to run super rich? Unplug the o2 sensor and the vacuum line to the FPR (plug the vacuum hose.)

Is it a single wire o2 sensor? It might be cooling off at idle. Have heard of people retrofitting heated ones.

From my experience with my 94 B4000, Ford computers are *really* into data from the o2 sensors, first.
 
Originally Posted By: eljefino
What if you force it to run super rich? Unplug the o2 sensor and the vacuum line to the FPR (plug the vacuum hose.)

Is it a single wire o2 sensor? It might be cooling off at idle. Have heard of people retrofitting heated ones.

From my experience with my 94 B4000, Ford computers are *really* into data from the o2 sensors, first.


I ran it with the 02 Sensor unplugged and nothing changed. I have to try plugging the FPR and actually driving it, that might help with the "lean" condition. I thought one of the techs mentioned ruling that out but I have not tried it myself. Sounds like a good idea and something worth trying by actually plugging it up and taking a drive. Thanks!

I think the 02 Sensor has two or three wires, its been a while I'd really have to look.
 
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If you unplug the O2, it will run in limp mode. You don't even need to spike the fuel pressure at that point. With the 02 unplugged, it should be a fair bit richer.

Maybe try that and see?
 
OVERK1LL we were posting at the same time. I already ran it with the 02 Sensor unplugged with no change.
 
I remember this issue. I remember suggesting unplugging the fpr just to see if it clears the stumble up. I also remember coming to the conclusion that since the throttle body base idle screw had been tampered with and it is a throttle tip-in issue that I think the computer is not able to control or juggle IAC on throttle tip in. Maybe if you could get another throttle body from the junkyard or off another car you could see if the issue clears up. I know you said the idle was properly readjusted, and the IAC and TPS are working properly, but I still think there might be an issue there.
 
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