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Toddred, Technician
Category: Car
Satisfied Customers: 57
Experience:  12 years owner of repair facility and 10 additional of tech expirience in auto, truck, heavy equip.
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1991 Isuzu Trooper: cylinder..injectors..fuel filter..air flow sensor

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1991 Isuzu Trooper 2.6l 4-cylinder, 2000 rpm idle and surging/hunting idle when warm, for a few moments, then settles down to a fast idle, 1500rpm, runs normally when cold. Have replaced all vacuum lines, have had injectors cleaned, timing set, new plugs and wires, new fuel filter, have cleaned air flow sensor.

When I remove foot from gas pedal while driving, then reapply slightly, car lurches forward, seems like some vacuum valve is not working properly. Have sprayed carburetor cleaner into all vacuum lines prior to replacing all hoses. It is annoying and dangerous and hard on transmission with abrupt reapplication of power no matter how softly I try to reapply power.

Thank you!
Your compression check reveals you have a lot of valve wear. 10 percent difference is the threshold and you are beyond it. With oil you only increase 5 lbs., so you should be okay on the lower end. When the engine is cold it runs on a pre set sort of setting and when the engine warms up the computer comes online and takes over the settings. So, you are looking at the current problem to be something feeding info to the computer. The best wat to find it is to put it on a scanner and watch live time readings. Not codes, but actual live time readings that are being sent to the computer. You might even find more that one component defective, like the o2 and ail flow meter.
Customer: replied 8 years ago.
Well, with more research, I am leaning towards a bad head gasket. I actually have water dripping out of my muffler, enough to be noticeable, not the tailpipe. I read a post here: Some of the comments seem to reflect my sysmptoms and the sollution seemed appropriate for this particular motor.
Your compression levels are pretty high to have a blown head gasket but you sure could have a cracked head. If you do, it could be doing the same things and look like a steam train in puffs out the tail pipe when warmed up. The heat can be making the crack open up more and force to o2 sensor to go nuts. It can't handle doing that much so you should also relace it if you decide to repair the car.
Customer: replied 8 years ago.
Cracked head, eh? Rats! I have a cooling system pressure tester. I was using it to find a coolant leak. It uses coolant but had a small hose leak. I started the motor with it hooked up and the system presurized to 15#'s. It immediately began moving up in pressure, close to the red zone and I turned-off the engine. This was yesterday. After your last response, I tried it again from zero. Within a minute of running, the gauge was up to 5#'s. Sounds like a cracked head, not just a head gasket, eh?

What happens when the O2 sensor is damaged? My gas mileage is about 17 mpg and I understand this vehicle should give more like 20-24 mpg.
<p>Sounds like a cracked head even more. </p><p> </p><p>The o2 sensor can only handle hot gasses and not steam. It will ever heat the sensor internals and never return to tolerance zero. Kind of like arching or stretching a spring and it never goes back to the old hight. So then the car's o2 info is always off or not working at all. The o2 senses oxegen and leans or richens the mixture to the right amount but if it is always off starting from start up it wigo back and forth trying to equalize and stabalize but it can't.</p>
Customer: replied 8 years ago.
Thanks for confirming my concerns.

Do any of these problems create the lurch when throttle is only slightly re-applied from a coasting state?

<p>Yes. Rich request from o2, throttle positioning sensor, and air flow meter can.</p><p> </p><p>Thanks,</p><p> </p><p>Todd</p><p> </p><p>P.S., don't forget to press accept if you value my answers and want me to get paid.</p>
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Customer: replied 8 years ago.
You bet! Just wanted to get enough driveability questions resolved before doing the head-job. I think I will get a new head. I understand they are beefier where the cracking occurs . . . and a new radiator . . . and a new O2 sensor . . . and new headlights, etc.
<p>Thank you for using If you need any help later on we will be here to help answer any questions you might have.</p><p> </p><p>Thanks again,</p><p> </p><p>Todd </p>