This truck is either going to drive me crazy or broke - maybe both.

1990 Dakota, base model 2WD long bed, 5 speed manual, TBI, no options of any kind. I bought it 15 years ago for $200, and have driven it almost every day since then.

Quick story to describe the problem:

Two weeks ago, I was driving up a slight incline, and changed lanes to pass an old lady who thought "speed limit 45" meant she should drive 20...so I downshifted from 3rd to 2nd, stepped on the gas, and...it fell flat. It didn't die, it didn't backfire, it just sputtered a bit and would not accelerate until I backed off the throttle. It never did this before.

I went to O'Reilly's and bought new plugs, plug wires, distributor cap, rotor, ignition pick up coil, and just for grits and shiggles, I also bought a new air filter. The last time I replaced any of those, it was 2009. (I know, but if it ain't broke...) Everything looked great - the plugs were worn but not badly, and they were beige, so I am pleased with that. The cap was in great shape, and the rotor was worn and was in need of replacement. The air filter, OTOH, was filthy.

I figured that replacing those parts would end the problem, but I was wrong. The next morning, the truck ran fantastic - not a problem at all - until it reached full operating temperature about three miles down the road. That's when I noticed it - no matter what gear I was in, I could only give half throttle before it started "bogging down". It seemed to happen when the RPM's were approaching the maximum torque range. I went back to O'Reilly's, told the guy what the truck was doing, and he began naming off everything I had done so far...then mentioned the fuel filter. Well...I know I have never changed it, and knowing the former owner of the truck, I doubt if he ever thought of changing it. So, I bought one, installed it, and immediately blew out the fuel line coming out of the filter. I replaced the line, and knowing the truck now had better fuel pressure, I took it out on a hilly country road...and it still bogged down when I put my foot to the floor.

The next day, I went to Auto Zone, and there just happened to be someone there with a Snap-On code reader. He plugged it in (I had already told him it was NOT throwing any code) and had me start her up. Both O2 sensors were reading good, all the sensors appeared to be within range - except when I revved the engine. Out of four times, only once did the timing advance properly. Now, on an old Chevy, I know that would mean the vacuum advance was shot, or the vacuum line was leaking. I assume the computer controls the advance on the Dakotas, so I decided to replace as many old, dried-out vacuum lines as I could find under the hood. Still no difference. I do hear a "pinging" under acceleration, like the timing was way off and I was using some really crappy gas (Yes, I thought of that, so I filled her with 93 octane AND put in a can of Sea Foam just in case the injector was dirty).

I replaced the temperature sensor behind the thermostat, thinking it might be causing the computer to be stuck in an open loop, but no luck there, either.

I'm not even going to consider the EGR valve as a potential problem, because the engine had a fuel leak during the Clinton administration that caused a small fire, and some of the vacuum lines were melted shut - and others were plugged - and the EGR valve hasn't worked in 20 years or more, but it didn't seem to make a difference in the way she ran.

Today, I noticed that at idle, you can barely hear a small variation in the idle speed. It will increase, return to normal, then decrease - I would guess by no more than 50 RPM either side of normal. I don't know if this has been going on in the past or not, I was really looking for symptoms today, and that's the reason I noticed.

And the final symptom is odd...I think the engine takes WAY to long to return to idle speed. While driving it tonight, I slowed down to make a turn, anticipating having to stop. As I was in 2nd gear, I did NOT downshift, but I did push in the clutch. It took about 7-8 seconds for the RPM's to drop from approximately 2800-3000 back to normal idle speed. When I got home, I put her in neutral and revved up the engine, and sure enough, it took 7-8 seconds to drop from moderately high RPM back to idle speed.

This is making me think it MIGHT be a dirty/defective idle air control valve. Of course, the guy at O'Reilly's is thinking it's the throttle position sensor.

While I still have some sanity and a little bit of cash left to buy parts, does ANYONE have any ideas of what my problem might be?

 

is the catalytic converter still on the truck, and intact? Sounds like it might be plugging up.

 

Yes, it is. I am thinking this MIGHT be the problem, but for some reason, I also think it isn't.

I used to have a Rampage years ago, and it acted different than this one when it plugged. To test the convertor on the Rampage, I used a 24 ounce framing hammer and a few wll placed hits to knock things loose inside.

It worked - the Rampage ran great after that. For about 10 miles, that is - then it plugged up bad enough it wouldn't run.

But would a plugged convertor cause the timing issue? (I'm not too familiar with anything newer than mid-70's engines. That happens when you're old, I guess...)

 

Probably not..... But, it will certainly complicate matters for the PCM, by not letting the exhaust out. Forced, constant, egr......

Easy test would be to remove the front O2 sensor from the exhaust, and see if the truck runs any better.

 

OK, you got me thinking...

Could this cause a "rattling" sound?

For quite a while, I have heard a "rattle" that sounds like a loose heat shield or bracket that seems to be worst at highway speeds. I have tried to find the source of the noise, and sometimes it sounds like it's under the hood, other times it sounds like it's from under the chassis. I haven't been able to find anything that could be causing the sound. I don't drive the truck on the highway very often - for the past couple of years, it's been almost 100% in town driving at speeds of 50 or less, and with the stereo on, I can't hear it. I thought about it, went out and took the truck for a short drive. It has the same rattle that it's had for months, but it sounds different somehow, more "muffled" than it was just a few weeks ago.

I'm thinking that if the "honeycomb" inside the convertor was loose for a while, it could have caused that "rattle", and being loose, it would be more susceptible to clogging.

If so, this would make a LOT of sense, with the backpressure building, it is going to send the computer into a fit trying to "fix" the problem.

 

Exactly. Your symptoms perfectly describe a failed cat.

 

Well....

It wasn't the cat. I dropped $300 for a new cat, muffler, and exhaust pipe this morning, and now I have a smoother running, quiet, rotten-egg smelling truck that loses power and pings like crazy when I accelerate.

So...in desperation, I changed the MAP sensor since I know that CAN affect the "pinging" situation.

No dice.

So - back to square 1.5.

Just to recap - new plugs, wires, cap, rotor, pickup coil, fuel filter, vacuum lines (well, most of them, anyway...some were capped off and not leaking), MAP sensor, and coolant temperature sensor, and catalytic convertor. Let's just say the O'Reilly family loves me right now.

I'm seriously starting to suspect the ECM...but something tells me that is not the problem.

 

Need to plug into it with a scanner, and have a look at what the PCM thinks is going on.

Was your coolant temp sensor a one-wire, or two-wire?

 

Two wire.

And I had it plugged into a Snap-On MT2500 type scanner last week.

All it showed was that the timing never changed when the engine was revved from idle. Everything else was looking normal.

 

Is timing controlled by a separate module on those?