mt4runner

'91 3.0, 5 speed.I've been chasing this miss for several months now and would appreciate any input. Did timing belt, water pump, etc, a while back. Timing is at 12 degrees, recently checked again and is still 12 degrees. New plugs, factory wires, rotor and cap. Doesn't happen all the time, but will occasionally run a little rough at start-up. Generally no miss or ruffness under acceleration as a rule, but every once in a great while it will. Some times it will idle and run perfectly. Checked for vacume leaks and didn't find any. Have changed fuel filter two weeks ago. Thought it might be bad gas, or water in the gas. Ran seafoam through the tank, thinking it might be a dirty or plugged injector. Thinking it still might be an injector, but not sure how to check specifically for that. I'm running out of ideas and open to any and all suggestions. BTW, this issue had developed shortly before I changed the T-belt.
Regards, Rick

TNRabbit

A miss is almost always caused by at least one of the following:

Vacuum leak (how did you check for leaks?)
Bad spark plug, incorrect gap, or wiring
Injector issue/wiring short

The wiring harnesses on these engines are old & tired after many years & miles. They are getting toasty/brittle which lead to a myriad of issues. I'm not saying that's your issue, but it's something to consider if you can't find the source of your miss. It's a B*TCH to locate a harness issue.

mt4runner

TNrabbit, to check for vacume leaks i used brake clean, and sprayed every where I could see hoses and some places that I couldn't. No change in idle or ingine speed. I did this when it reached full operating temp.

mt4runner

I should probably clarify that this miss is just a stutter or slight roughness. not anything like a fouled plug or plug wire disconnected.

TNRabbit

Did you account for the vacuum lines on the passenger fender well TVVs? What about the two going to the front diff?

I had the same stutter for 2 years when I discovered a HUGE hole on the underside of the AC idle up valve lines.

mt4runner

Passenger side, yes. Front diff, no, and not sure exactly where ac idle up lines are. One thing I have had ever since owning this(5 years) happens in cruise control, decending a hill. Often it will start to buck a bit. Not all the time ,but frequently. It'll jerk 4 or 6 or ? however many times until throttle is applied at the end of the grade. I've often thought that this might be a vacuum related issue as well, but could not find a cause.

mt4runner

Last night, took a drive for 20 miles, slight miss all the way, enough that power loss on small hills was apparent. After finishing my business, headed home(approx. 3 hrs. later) ran like a champ.Tonight i pulled sparkplug wires while it was running a little rough. #5 had the least effect. Still at a loss, hope to have a little time this weekend to work on it some more.