Hello,
I have a 1990 pickup. The engine is from a 1992. It has been having trouble the past few days, and is progressively getting worse.

It idles good most (but not all) of the time. When I press the pedal, the rpms slow down, it acts like it wants to die, and almost sputters. I can get it very gradually up to about 2500 rpm, then it is fine.

Any thoughts? Fuel pump, or TPS?

 

It was running as good as it gets and just slowly got worse.

Got worse boom just like that??

Bad tank of gas or ran your tank real low and sucked more dirt into the filter??

This Ethanol gas is hard on filters when did you change yours last??

 

The fuel filter is only a couple montgs old.

It was running fine until about a week ago. I did wonder if it was bad gas, so I added gas treatment but it didn't help and is getting worse.

It seems fine on the highway as far as I can tell. It's in town where this happens. Sonetimes it's fine but then some times it does this. And it is doing it more and more often, and more severly than it was at first.

 

One other thing I noticed - Lately I have been able to hear the fuel pump running. What I mean is, if you stand near the gas tank, you can hear a high-pitched whine. I never noticed it before a month or so ago. I'm not sure if this is anything of concern or not.

I'm not sure about the fuel pump though, because it runs fine once you hit 2500 rpm.

 

When was your last major tune up??

These engines need fuel Air and spark.

What does the throttle body look like does it need cleaned??

Is the check engine light on??

When things start running poor pull the spark plugs see what they tell you.

 

The check engine light is not on. I wish it was!

This afternoon I removed and tested the TPS. It tested fine, so I put it back. After that, I drove to the store. On the way to the store, the truck ran fine - totally normal. When I came out and started it, it was idling lower and running poorly again. This continued most of the way home, then started to get a little better.

The way the issues are off and on, it makes me wonder if it's a sensor going bad. It kind of reminds me of when I had a crankshaft sensor go bad on a '97 Wrangler.

 

Almost sounds like your not getting enough fuel either filter is clogged from a tank of bad gas.

The pump is on the way out

Do you have a fuel pressure gauge or know how to hook one up??

You have checked the air filter??

 

I agree that it does seem like something with the air / fuel amounts. But it's weird how sometimes it seems fine.

I do not have a fuel pressure gauge. That would be really useful though. Maybe I should look into that.

 

In my experience, when the TPS is maladjusted it will run fine right upon plugging it back in. Then once the engine has been shut off and you try to turn it on again, all heck breaks loose.

When you put the TPS back on, did you use feeler gauges and a multimeter to get it adjusted perfectly? If not that will most certainly cause issues. It takes about half a hair's width of adjustment to get it right, very very sensitive.

You can reproduce the problem by resetting ECU (pull ECU fuse or disconnect b+ for 30mins) and if it runs well again it is probably just TPS. In fact, the symptoms you describe are telltale of TPS being out of adjustment. When mine was wrong, I would rev slowly and when it hit 1500rpms it dropped back to 1000, if I held the throttle steady it would do this over and over. Only if I revved fast and over 2500 or so would it continue to reach farther.

Hope this helps

 

I'm old school carbureted so that's the way I think. To me it sounds like an EGR valve that mostly doesn't close completely, like a piece of carbon stuck in the valve. At 2500 RPM or higher the engine is fine with it, at lower RPM's it is running lean due to the introduction of exhaust gasses into the intake.
It is reminiscent of fuel starvation too, but the EGR remaining partially open would give that effect.

 

i agree with checking the adjustment of the tps. if it acts like it has power above 2500, that doesn't sound like its starving for fuel. the way you describe your issue, i would check the tps adjustment first. noisy fuel pump doesn't sound good though, all the yota's ive heard are quiet.

 

I almost wanna say TPS. because that's what mine did before I got it fixed. check over all the basics first though, can't hurt even if it turns up nothing

 

http://4crawler.com/4x4/CheapTricks/...#TPSAdjustment

Feeler gauges are a must! Unless you have a digital caliper, you can use a series of credit cards & sheets of paper to get the required offsets (what I did). I bought feeler gauges the day after, it would have been way easier with them.

 

Thanks guys for your ideas. I did properly adjust the TPS with feeler gauges and an ohm meter. I also disconnected the b+ today for about 4 hours, but when I hooked it up the truck still ran poorly.

I called my mechanic today and he was thinking more along the lines of ethanol build up or bad gas. He suggested adding K100 to the gas. I got some after work and drove around a good bit. It's not perfect, but it's about 90% better. I'm starting to wonder if I got some bad gas.

 

Well, I'm on my second tank of gas with the K100 added. I also replaced the spark plugs about a week ago. The truck runs better than it did, but this problem is not out of it's system. When I drive it, it comes and goes. I'd say 75% of the time it runs normal, but the other 25% it sputters and tries to die out when I press the gas pedal. If it's really bad, it will even backfire. Nice, huh?

I do wonder if it's the TPS, but I did test it with an ohm meter and it tested fine. But then again, the problem doesn't happen 100% of the time.

I do also wonder if it's something with fuel pressure or the EGR system.

When I was younger, I'd just start buying parts until the right one fixed it. However, I've learned that it can sometimes be cheaper to just take the truck to a mechanic and pay for a little labor, and only buy one part. So I might end up doing that, but if anyone has any other thoughts, please share them.

Thanks,

 

Sounds like you're on your way to fixing it. The bad thing about the ethanol is it gels up and turns to sugar because of the corn content, and it covers everything in the fuel system: fuel pump strainer, fuel filter, fuel rail, fuel injectors. I would try running some Seafoam through the system to clean it as best as possible, and do it like 2-3 times upon fill-up. Then replace the fuel filter just to be sure.

If you can hear it whine, though, it does sound like the fuel pump is struggling a bit too hard...

 

I have a very similar issue. Fires poorly at low rpms especially shifting 2nd gear. I changed spark plugs, air filter and fuel filter. Seems better but still there. When I first started it after changing the fuel filter it ran almost perfect. I thought i had found the issue (fuel filter was filled with dirty brownish gas with impurities resembling pepper flakes) but then i parked it and had trouble with the reverse gear. Woke up this morning toke a shower and i was about to take it for a ride and i was greeted with a puddle of diff oil on the ground... guess the firing issue is going to have to wait.
I'm certainly going to be monitoring this thread! So if the problem is solved, I would appreciate it if you would post what you have done to fix the issue!

 

Yes, I will let you know what the problem was when it is fixed. It's at my mechanic's now. May The Lord give him wisdom.

 

Well...the truck was at the mechanic's the past few days. He poked around some, and noticed the check engine light was not coming on at all. I took the truck home and discovered the bulb for it was missing. I installed a bulb, and was able to read the codes. It gave me four codes: 21 (O2 sensor), 24 (intake air temp), 31 (airflow meter), and 52 (knock sensor). So that's where we are at now. Sounds expensive, but glad to be making progress.

 

I'd tackle 24 and 31 first. They both have to do with the VAFM (vane air flow sensor), and most likely the connection is bad, or someone tried to remove it the wrong way and broke the traces off the PC board inside. Without the VAFM working, the ECU is just guessing at proper injector timing, so it's not going to run well. There are some VAFM diagnostics in the FSM that you should perform with a multimeter. Check this page http://personal.utulsa.edu/~nathan-b...33volumeai.pdf

The right way to get into the VAFM, if you need to open it up, is to cut the RTV around the top plastic cover and remove that. Then you can see the circuit board and wiper and the mechanics of the device. Do NOT try to remove the screws holding the connector in and pull on the connector housing. That will destroy the connections to the PC board.

General forum wisdom is that the code 52 is usually a bad pigtail wire going to the knock sensor. The knock sensors themselves tend to be pretty rugged, but the wiring is in a hostile environment and is eventually prone to failure.

When you get those three codes fixed, see what the o2 sensor does. It might be time to replace it, anyway.