Questions and Answers
Second: The owner has taken the vehicle to a shop and replaced these components: EGR valve, Thermal valve (name given by manual, valve that controls hot & cold air coming into throttle body) and the O2 sensor.
Isuzu Trooper Idle Problem
Q. Vincent, I have a question about a 1988 Isuzu Trooper, 2.6 liter fuel injection, manual transmission.
First, symptom: at peak operating temperature at idle the engine revs up and down between 900 and 1,200 rpm.
I was thinking it was an air leak, are some hoses on backwards maybe? Any help would be very grateful. Do you need more info?
A. If there's one thing I've learned over the years it's never to try sticking a piano up your nose. No, wait that's not it.
The first thing you need to do is to check the basics. Many a part has been needlessly replaced because of a broken vacuum line or loose connection. My neighbor had a Chevy Blazer with a big four barrel on it and he was ready to replace it because it was not running right. I saw him in the driveway playing with it and the new carb was sitting on the ground. I walked over and asked him what was up and he told me what it was doing.
I had him start it up for me and I immediately heard a vacuum leak. I reached in and found a cracked PVC hose. I told him there was his problem. He took the carb back and got a new hose and it ran like a dream again.
So check the basics, make sure the lines are in good shape and hooked up. If you have a doubt as to where the lines are supposed to be, get a vacuum diagram and check each one. Check the electrical connectors and make sure they are clean and tight. Most connectors have a latch or lock that holds them together. Sometimes they get brittle and break; or corrosion builds up inside them.
If it's been a while since the plugs, distributor cap, rotor and wires have been changed, replace them. It never hurts to do that and you will be sure the problem is not there. Once the basics have been checked, then pull any DTCs from the computer. This will point you in the right direction.
A problem like yours can have several different causes, some of which can be the result of one of the basics.