My brain hurts!

My 93 Trooper has acquired a habit of chugging below 3000rpm after about 2 min of running from a cold start. I racked my brain, swapped sensors, checked the timing, cleaned the EGR tube and valve, changed all the vacuum lines, tried high octane fuel for a couple tanks, and ran seafoam through a tank of fuel. All as I was able to over the last 6 months or so.

Last fall it was acting similar but since I was working I took it to the Saturn/Saab dealership since they used to be the local Isuzu dealership to get looked at. they discovered that the fuel pump was only producing about 8psi, and the filter needed to be changed. $1200 later she ran like a dream, for a month then slowly started developing the current problem.

For two or so min after I start it from sitting over night it runs like a dream with tons of power. After that getting moving in first is a bit of a feather the gas and clutch job. Once I get up over about 2500rpm or so it takes off with tons of power again.

Occasionally when I let off the gas the rpm will drop to about 200 and sputter and cough for a few seconds before rising back to 750 or dying.

In the last week it has developed another symptom, black fuel filled exhaust for the first little bit after I start it, which goes away as the converter warms up..

One thing that I haven't done yet is to hook up a vacuum gauge in the cab to monitor the vacuum while driving. I don't have a vacuum gauge to do this at this time either.

The CE light hasn't come on, it didn't come on when the fuel pump wasn't working either though.

I need some help with this, any and all suggestions would be greatly appreciated. Thanks so much.

One possibility that comes to mind is the O2 sensor. Sounds like it is running alright in open loop mode, but when it goes to closed loop as it warms up it starts taking info from the sensors and if the sensor is crap it will run like it.

When it is warm, how does it do on wide open throttle? That is another circumstance that is open loop. If it takes off fine when you floor it but runs crappy the rest of the time when warm it points to a sensor problem.

I replaced the O2 sensor last fall when the light came on so I have been hoping that that isn't the culprit.

I got the same suggestion over on Planetisuzoo as well as checking the plugs and wires, which I had over looked because I had them been replaced right after I got it in the fall of 07.

I just got plugs and wires today, got them replaced this evening, the shop that I had do the "tune up" after I bought the truck failed to put dielectric grease on the plug ends boots. There was a lot of burns and scorch makes on the spark plugs and the wire ends.

There were a few other things that have become problems that the shop I took it to was supposed to have checked... I now remember why I like to do my own work, it gets done right!

I will update tomorrow after I take her for a drive.

I was going to suggest plug wires except for it smoothing out at higher rpm. But it is certainly worth a look. Shortly after I bought my Trooper a cylinder started missing, drove home from a long trip on 5 cylinders (long story). When I pulled the plug wires there was a huge carbon track on one of the plug tubes. New plug wires cured that.

Sometimes you can run it in the dark and look for arcs, but the way the plugs are buried in the valve covers the arcing can be hidden.

I put about 120km on it yesterday and it would appear that the plugs and wires were not the whole problem. My gas mileage is doing better than it was before though so it was something that needed to be done. I have the old O2 sensor that I may as well swap in to see if it makes a difference. It seemed in pretty good condition when I took it out, and it was before this problem that I changed it. The condition of the one that is in right now may shed some light on the whole situation too.

Good idea.

I got the O2 sensor changed back to the old one and after driving a few days it seems to be running a bit better but still not even close to great.


Old one on the left, replacement on the right. The replacement only has ports on the tip, nothing on the sides.

I picked up a intake gasket set cus they are really inexpensive. I am going to wait to dig into that until I can check the running vacuum levels and weather it is bobbing around all weird.

I suppose that I should get a fuel pressure gauge too so that I can check that the regulator isn't pooched.

Any other suggestions as to what to look for that I may be totally missing?

I got a vacuum gauge all hooked up it doesn't jump around at all, it sits at a very steady 19in at idle. When I apply just a bit of throttle with with load (accelerating) though it drops right to zero and slowly rides back up, when it chugs it jumps up and down with each jolt. It also seems to be starving for air at times too at which point it kinda wiggles the needle back and forth. When I apply a little throttle while parked the needle drops a bit then rises to about 23in. I have it hooked to the line that goes to the fuel pressure reg.

That is what I have observed so far, that may or may not shed some light on my situation.

I have gone over all the vacuum lines again, I have sprayed all the joints in the intake system with intake system cleaner without so much as a stutter in the idle rpm.

I checked both of the VSV's, both are working correctly.

I was thinking to check that the signal is getting to the ECU from the O2 sensor, any pointers on that would be appreciated. I just read somewhere that the ECU doesnt listen to the O2 sensor till it is warmed up (about 2 min) which would be at right about the time that it starts acting up after a cold start.

I am sure I have seen how to check the fuel pressure reg but I haven't been able to find it in either of the Chiltons or Haynes manuals. Any pointers on that would be appreciated as well.

I have a 7 hour drive to Fairmont Hot Springs on Thursday, I would love to get it working a bit better before then.

Thanks for all the help so far and the help to come.

Bump

So I finally had a day off with warm enough temperatures and enough time to get out to the garage to test the fuel system.

Fuel Pressure regulator was no longer responding to vacuum variations correctly. When high vacuum was applied it would sit at 26psi steady which is okay but when the vacuum was removed the pressure would drop a bit and flutter instead of rising to between 35-40psi. Changed it out and all the tests came up properly.

Took it for a couple drives last night and WOW I can actually accelerate steadily, start in 2nd, and cruise at low speed in a high gear without doing some fancy clutch and gas pedal fluttering.

[color:"red"]Thanks for all the help![/color] I am still going to change the fuel filter and add some injector cleaner for good measure, the injectors may still be gummed up from the low psi injection at low vacuum.