My99

I drive my 86 22RE regularly, but not on extended trips. Most of what I do is around town at relative low speed or with constant speed fluctuation. Today I took the old gal on a relatively lengthy road trip. About 30 miles into the trip I noticed the first occurance of the issue I described as what feels like a dead spot in the TPS. I am cruising along in 5th gear at 70mph, just under 3k rpm. All of a sudden its as though I hit what is known as fuel cut in the turbo world... where essentially the ecu pulls shuts down all (or what feels like all) combustion operations for a split second. It comes back, but it is particularly annoying at 70mph. It seems to be more likely to happen under extra light load - such as when going down a hill or letting up for traffic, etc. Never does it while under load. It feels like it happens at about the same throttle position.

So my question and what I am looking for input on is what are the most likely suspects? I instantly thought TPS. But I suppose it could also be the air flow meter? I do not really suspect the fuel system, as it only occurs under very light load and only during cruise.

Taking all input!

alltracman78

Easy way to check for a dead spot in either.
Find the VS [AFM signal wire] and VTA [TPS signal wire] at the ECU connectors.
Find a good ground for your multimeter negative lead [E1/E2 will work great if you can't find anything else].
Key on, everything hooked up/connected.
Positive lead in the VS signal wire [you can slide it into the rear of the connector so it contacts the metal pin on the end of the wire]. Have someone SLOWLY move the AFM flapper from fully closed to fully open. As the flapper moves open the voltage should smoothy drop from ~ 5 V to 0 V.
Positive lead in the VTA signal wire. Have someone SLOWLY press the gas pedal to the floor. As the pedal goes down the signal should smoothly change.

My99

Appreciate the recommendation, and I can certainly do that. Challenge is that this is a very sporadic issue, and I feel my ability to reproduce it may be low. During my 200 mile drive, it crept up maybe 15 times, and I was at a pretty constant speed most of the time.

alltracman78

Well, intermittent problems are a pain to diagnose.

It does sound more like the AFM [if it is a dead spot] because it's only at hiway speeds [correct me if I'm wrong]. TPS should be in any gear. It also shouldn't be effected by load.
The AFM will be more effected by load. No load it will usually move very slow if at all, high load it's going to be opening.
So if you do have a dead spot it's more apt to "sit" on it while you're under no/low load.

If it is the AFM it is probably at nearly open [somewhere under 1.0 V] because of the speed you're at. So maybe focus on that, see if you can get anything out of it when you test it.


*Now that I'm thinking about this [jumped right to what you said, didn't really think about this], when it happens have you just let off the throttle? So it snaps closed [AFM will to in that case]?
Or are you at steady throttle angle?*

My99

I'm at steady throttle when this happens. Just cruising along at 65-70. Like I said, seems more prone to do it at lighter throttle, such as when backing off for traffic or on hill descents. It very well may do it in other gears, but probably the high speed and aerodynamics of a brick make it much more apparent at higher speeds.

alltracman78

If you can, hook up your multimeter to the ECU like I said, hooking the postive to the VS wire. Leave it hooked up as you drive around every day.
Keep an eye on the signal and see where the signal is when it does this, and what the signal does when it does this.

My99

On the way home it got very annoying to the point a I pulled over and disconnected the TPS in hopes that would solve my issues. No luck though.. Still getting the bucking at highway cruise under light throttle. <scratches head>

854x4

A multimeter might not catch the glitch you might need to scope it. I have seen where the tps wont spring back to its position meaning if the have the tps in you hand you should be able to turn the tps and let go it should spring all the way back, if it sticks or doesnt have a tight spring it can cause this problem.

alltracman78

It may not.
An analog volt meter would be best, but they aren't as common, and it's worth a try with the multimeter.
That's also why I suggested driving around with it hooked up, more chance of seeing the problem [if that's what it is].

Have you tried what I suggested?

I doubt it's your TPS because TPS would be throttle angle related [meaning in every gear].
Does it do it in 4th when you back off?
What about 3rd?

My99

Does it in every gear, and I even put it in neutral when this started during cruise and found it would do it even in neutral. The RPMs would act erratic even as the throttle was held steady. It's like a dead-ish spot.

cjserio

I couldn't believe my eyes when I saw this thread because my truck does the same thing. If i have my foot on the gas at JUST the right location, the truck will VIOLENTLY miss. It feels like I hit a brick wall. It does it in all gears and in neutral. I can replicate it 100% of the time if i move my foot to the tiniest spot.

In my case it IS the TPS. I verified it with a meter. The value on the meter is slowly increasing in response to my foot on the throttle and then all of a sudden at the same spot it just comes back with a bogus value. It's like a dirty spot in the TPS. I bought a new TPS from Rock Auto and the new one does the same thing though it's less "wide" of a spot if that makes sense.

Are these devices made that poorly that I could have gotten another bad one?

My99

Interesting... FWIW I have a spare (which I tested as bad a few years back) TPS I guess I should test in addition to the one that's on the truck right now.

Is there any other input that could be causing this irregularity? IE - another component such as the AFM that sends a signal that interrupts or otherwise is taken in conjunction with the actual throttle position to help apply the correct fuel for the application.

I ask, because with the TPS unplugged completely, this issue persisted... And significantly impacted fuel economy in the negative direction.

And all trac- no I haven't had a chance to try your method. I was out of town and despite having a set of tools onboard at all times, a multimeter isn't part of the set.

My99

Update: This afternoon I finally had a few minutes to break out the volt meter and verify some things. I tested the TPS, and it seemed to check out fine. So then I went to the AFM and I found an issue.

Mad props to the author of this site, I can't tell you how valuable his information has been: http://www.4crawler.com/4x4/CheapTricks/AFM/index.shtml

Anyway, static test number one (E2-Vs) failed miserably. I had to turn up the ohm meter to the highest (millions?) setting in order for it to read some legible value whereas the expected value is to be 20-400 ohms. It so happened that the other AFM I had laying around (swapped a while back for another reason) and so I tested that one and everything tested out fine. So I swapped it back in. I'm not sure what specifically E2-Vs being out of whack will do... but that's the only issue I found in doing the diagnosis today. I'll post back when I have results from a road test to let you know if this solved the issue.

abecedarian

You should thank 4Crawler here on Yotatech.
He's got his own website for working on our trucks!
That should say alot.
We are truly grateful he's here too.
His information has helped me make sense of things.

My99

Werd, Abe. And to put a close to this, I have taken multiple road trips since putting my "old" AFM on there and have had no issues since. Problem solved.