Having weird power issues with my '88 2.9

[DisturbedMXer8]

I've looked through these forums and can't quite find any info on my current issue, so if anyone can help me out on this please let me know.

Here's the thing: I have intermittent power loss below 2.8k-3k RPM. What I mean by that, is that any time I apply more than 1/3 throttle or any time I add weight (eg. another person in the truck with me), it acts like it has less power. So, for example, I get on the throttle pretty good to get on the freeway at a good speed: once I get in third gear, around 40 MPH, from idle to around 2.8k RPM, it acts and sounds like it's choking, as if I were trying to tow 20,000 lbs with it (which I'm not). Then, the second it hits about 3k RPM, it instantly cleans right up.

I checked codes (KOEO, the whole deal), nothing. My first suspicion was the timing, however I recently had it checked and it's at spec. So later, I hooked it up to a scope and noticed the firing lines all over the place. So, I replaced the Distributor, cap and rotor, TFI Module, and the coil. Still have the problem. I really have no idea what else could cause this.

Here's the weirdest part... it sometimes gets better, sometimes gets worse. I used to be able to tow a 500lb trailer full of bikes, gas, and gear up hills no prob. Now it struggles with just a passenger riding with me.

Anyone have any ideas? Any help would be greatly appreciated.

Just to clarify, I've already changed plugs (and wires), air filter, fuel filter, and even replaced a pressure regulator that went bad on me. Like I said, any help would be greatly appreciated.

 

[IMenriched]

I've looked through these forums and can't quite find any info on my current issue, so if anyone can help me out on this please let me know.

Here's the thing: I have intermittent power loss below 2.8k-3k RPM. What I mean by that, is that any time I apply more than 1/3 throttle or any time I add weight (eg. another person in the truck with me), it acts like it has less power. So, for example, I get on the throttle pretty good to get on the freeway at a good speed: once I get in third gear, around 40 MPH, from idle to around 2.8k RPM, it acts and sounds like it's choking, as if I were trying to tow 20,000 lbs with it (which I'm not). Then, the second it hits about 3k RPM, it instantly cleans right up.

I checked codes (KOEO, the whole deal), nothing. My first suspicion was the timing, however I recently had it checked and it's at spec. So later, I hooked it up to a scope and noticed the firing lines all over the place. So, I replaced the Distributor, cap and rotor, TFI Module, and the coil. Still have the problem. I really have no idea what else could cause this.

Here's the weirdest part... it sometimes gets better, sometimes gets worse. I used to be able to tow a 500lb trailer full of bikes, gas, and gear up hills no prob. Now it struggles with just a passenger riding with me.

Anyone have any ideas? Any help would be greatly appreciated.

Just to clarify, I've already changed plugs (and wires), air filter, fuel filter, and even replaced a pressure regulator that went bad on me. Like I said, any help would be greatly appreciated.

might be time to have the compression checked.
Have you checked for vacuum leaks??

also have you had the fuel pessure checked?

 

[holyford86]

when was the last time you changed the fuel filter? check your grounds to the engine and computer (there are three to the computer, one at the battery and 2 behind the passenger side kick panel.) Make sure that the orange ground wire to the back of the passenger side head is intact as well. Make sure that your wiring to the o2 sensor is good as well.

 

[DisturbedMXer8]

I changed the fuel filter a few months ago, it's practically brand new. Same for the 02 sensor, did them the same day.

Also, fuel pressure is good. Sits at around 32 psi at idle, goes up to about 42-45 when I hit the throttle.

Haven't checked for vacuum leaks, don't think I have the means to. Any suggestions on how I can do that?

I never really considered compression. I don't think that bad compression would only affect it below 3k RPM and then be fine above it. I'll check it next time I get the chance, though.

 

[IMenriched]

I changed the fuel filter a few months ago, it's practically brand new. Same for the 02 sensor, did them the same day.

Also, fuel pressure is good. Sits at around 32 psi at idle, goes up to about 42-45 when I hit the throttle.

Haven't checked for vacuum leaks, don't think I have the means to. Any suggestions on how I can do that?

I never really considered compression. I don't think that bad compression would only affect it below 3k RPM and then be fine above it. I'll check it next time I get the chance, though.

for the vacuum leaks...you can check the vacuum with a gauge.
http://www.therangerstation.com/Magazine/Summer2003/VacuumLeaks.htm

a quick check is to spray a can of carb cleaner/ ether, some thing flammable, as it is idling & warm ...spray around all vacuum lines and intake gasket seems....if the engine changes in RPM. there is a suspect area. Note-stay away from the exhaust area

 

[bronconcious]

Make sure that your wiring to the o2 sensor is good as well.

What is the route of the o2 wires on a 2.9L?

 

[DisturbedMXer8]

I was wondering, could bad exhaust (cracked/clogged/restricted) cause this type of problem? Like maybe a melted cat? I haven't checked any of my exhaust yet, was wondering if it's even worth my time?

Also? How would a bad ground to my computer cause power issues but still allow it to run? And wouldn't faulty wiring to the O2 cause the "check engine" light to come on? Not trying to challenge anyone, I genuinely want to learn, haha...

 

[IMenriched]

I was wondering, could bad exhaust (cracked/clogged/restricted) cause this type of problem? Like maybe a melted cat? I haven't checked any of my exhaust yet, was wondering if it's even worth my time?

Also? How would a bad ground to my computer cause power issues but still allow it to run? And wouldn't faulty wiring to the O2 cause the "check engine" light to come on? Not trying to challenge anyone, I genuinely want to learn, haha...

As holyford mentioned checking for bad ground...is pretty legitimate. Personally I've gain more in runnability by making sure grounds are up to the task.

for example....have you ever been driving at night, and noticed another vehicle with a intermittent tail light....basically it is a bad/poor ground issue with the tail light. The same thing happens with a ignition and a iffy ground for it...possibly in your case, in a certain RPM range where it flares up.

as far as bad wires for the O2 sensor...yes they would throw a CEL, because the ECM isn't receiving a signal from the O2 sensor.

For the exhaust causing a fade in power...typically that rears it ugly head after things are warmed up and across the whole RPM range as opposed to just a selected range, especially with Catalytic Converters.

I realize you recently changed the fuel filter....but sometimes with the fuel filter it can replug rapidly ..because the fuel system has once again better flow, which now moves more crud/sediment, which the new filter now catches. If your servicing the fuel filter regularly in the past...then it prolly is fine.

Have you recently changed your tires size...increased?

 

[DisturbedMXer8]

Ok, I see what you mean with the grounds and stuff. I know electrical pretty well, but didn't think a bad ground would cause this type of problem. I'll definitely take a look into it. I assume that since I'm not getting a MIL, that my O2 wiring is good, then? Like I said, it's a brand new O2.

As far as the history of fuel filters, I couldn't say for sure. This truck pretty much sat around for 9 years or so before I got it. Might explain why problems are coming up now that I've been driving it 80 miles a day =P.

And no, stock/original sized tires and rims. Stock gearing, as well.

One thing I noticed: It only happens when I get hard on the throttle, not if I just slowly increase speed. Also, if I'm just cruising at 2k RPM at 65 MPH or so and then I slowly apply more throttle, my RPM stays the same and I can hear it start getting worse as I apply more throttle, almost as if my throttle is opening and allowing more air in, but the fuel isn't increasing, therefore leaning it way out (which would explain why it gets much hotter when I hit hills). RPM won't increase until it shifts down and speeds up, or if I just go nice and slow like I mentioned. Don't know if this is understandable the way I explained it, or if it's a "you gotta be there" type deal...

 

[IMenriched]

Ok, I see what you mean with the grounds and stuff. I know electrical pretty well, but didn't think a bad ground would cause this type of problem. I'll definitely take a look into it. I assume that since I'm not getting a MIL, that my O2 wiring is good, then? Like I said, it's a brand new O2.

As far as the history of fuel filters, I couldn't say for sure. This truck pretty much sat around for 9 years or so before I got it. Might explain why problems are coming up now that I've been driving it 80 miles a day =P.

And no, stock/original sized tires and rims. Stock gearing, as well.

One thing I noticed: It only happens when I get hard on the throttle, not if I just slowly increase speed. Also, if I'm just cruising at 2k RPM at 65 MPH or so and then I slowly apply more throttle, my RPM stays the same and I can hear it start getting worse as I apply more throttle, almost as if my throttle is opening and allowing more air in, but the fuel isn't increasing, therefore leaning it way out (which would explain why it gets much hotter when I hit hills). RPM won't increase until it shifts down and speeds up, or if I just go nice and slow like I mentioned. Don't know if this is understandable the way I explained it, or if it's a "you gotta be there" type deal...

Might be more of of MAP sensor issue not picking up the change in manifold pressure??? just tossing out thoughts

 

[piotrsko]

I had a dirty throttle position sensor do sort of this.

VOM'd the sensor per the tech library and it skipped. Put on a new one and that problem went away, still working on others.

Kid is tired of crawling under car, now wants to go to college.

 

[DisturbedMXer8]

I'm glad you said that, the MAP was one of my suspicions. I'll look up the pin point tests for it and check it out. I also just went and picked up a vacuum gauge, so I'll be testing it a little later today. I'll post up here with what I find out.

I'll definitely check the TPS as well. Last time I checked with my Ohmmeter, it looked good. But who knows, second check can't hurt.

 

[DisturbedMXer8]

UPDATE: Ok, I checked my vacuum and it looks ok. It idles at about 17-18, but when I snap the throttle it drops to below 0, back up to 23 or so, then goes back to 17-18. So according to the vacuum leak page, that's just showing maybe slightly low compression and/or I need an oil change. Would these readings cause my problem, though? My vacuum overall looks slightly low, but very steady.

Also, I can't seem to find a test for the MAP. Is it possible to test with a DMM?

 

[DisturbedMXer8]

So, I can't find any tests for my MAP sensor online. I think I'm going to just grab a new one and see if it helps. It sure can't hurt since the one on there is 22 years old, and I'm running out of ideas here. The truck passes smog, has all new ignition system, good fuel pressure, good vacuum, brand new O2 sensor, good grounds (at least from what I can see)... at this point, it HAS to be a sensor, and Manifold Pressure Sensor issues would make sense to me. If it doesn't work, I'll look up tests for the rest of my sensors at school and check them ASAP. I'm running out of patience, though, haha... any opinions on the subject would be much appreciated.

 

[IMenriched]

UPDATE: Ok, I checked my vacuum and it looks ok. It idles at about 17-18, but when I snap the throttle it drops to below 0, back up to 23 or so, then goes back to 17-18. So according to the vacuum leak page, that's just showing maybe slightly low compression and/or I need an oil change. Would these readings cause my problem, though? My vacuum overall looks slightly low, but very steady.

Also, I can't seem to find a test for the MAP. Is it possible to test with a DMM?

vacuum numbers are not terribly out of wack. might want to look over /replace those hard plastic lines....over time they get very brittle.

yes you can use DMM on everything but the TPS....with the TPS you really need the needle sweep action of a analog meter...it's far more easliy seen/noticed when there is hesitation in voltage as one actuates the TPS