Hi. I have been trying to solve what's with my dakota 1991 3.9TBI and done lot of different checks. It has rough idle, backfiring, missfiring, popping exchaust, hesitation, hanging idle. But first id like to know what is the valve clearance of this particular engine. When #1 cyl set TDC of compression cycle, shouldn't it have valves fully closed and clearances at maximum. Now it has 0 for intake and 0.05mm (0,0019685") for exchaust when cold engine. Does this engine have hydraulic lifters? Okay. To answer to myself. It does so clearance cannot be measured. Right?. Also did leak-down and pressure test:

Cyl1, 91% 190PSI.
Cyl2, 86% 182PSI.
Cyl3, 85% 193PSI.
Cyl4, 85% 182PSI.
Cyl5, 91% 179PSI.
Cyl6, 89% 185PSI.

Distributor cap, rotor, plugs and hall effect sensor has been replaced. TPS has no erratic behaviour, gets 4,95V in. MAP voltage changes smoothly when ingreasing vacuum manually. Do not own gauge so don't know how much vacuum. Puts 4,5v out when ingnition on without running. O2 sensor, yes i know it doesn't matter while proplem exists when open loop, anyway it seems to give somewhat correct readings until suddenly jumps off scale. Thinking thats a symptom, not reason of rich a/f ratio. Temp sensor does change idle if wire taken off while warm engine. Ingnition timing has been checked. IAC cleaned, checked and now taken off because annoying hanging idle. Smoke tested vacuum leaks where found leaking EGR which is now plugged.

I'm not really experienced to analyze fuel stream coming out of injectors. Amount seems to be same, no leaking, pattern ok. Fuel pressure is 14-15 PSI. No fast dropping.

 

I did my o2 sensor and it cured my horrible MPG and lack of power. The temp sensor helped as well. Those two are NEVER replaced. I suggest it.