Sluggish under load. '94 5 speed.

Hi guys,

I have been searching but cannot find a thread that matches exactly what I am looking for.

I have a 94, 4 door, 4wd, 4.0, 5-spd. and the power under heavy acceleration is not that good. The low throttle input (maybe 20% or less) feels good and it has good torque, however I can feel the engine lose a bit of power when it gets past that point. It is like this most of the time, but every once in a while, it seems to run a lot better. Sometimes it will bog a bit, then take off with what I assume is full power. So, whatever is failing does seem to work sometimes.

The only other odd thing it does, is that when its cold, it will buck a bit at low throttle input (parking lot types of situations). After a minute or two, it goes away.

Gas mileage is 16-18mpg all the time. It has a 3" lift and 30" tires.

So far it has been tuned up, fuel filter (~20k ago), tps sensor replaced, map sensor tested, injectors cleaned, APN header, new cat (old one fell apart inside), O2 sensor...I think thats about it. I tested the CPS as well. And cleaned up the ground wires.

Hopefully someone has some suggestions. I was thinking coolant temp sensor might be the culprit.

Thanks.
Chad :cheers:

Assuming no check engine light. By tuned up, can we assume that means plugs, wires, cap, rotor?

Coolant temp sensor can be tested for resistance if so desired. Checking fuel pressure and comparing to spec is something that is easily done and never a bad idea as a matter of course.

Check temp sensor plug connection, next check for proper resistance at temperature. If you search a little, the ohm values have been posted.

Tune up included cap, rotor, wires, plugs. No check engine light.

I actually tested the fuel pressure, but I have not done so while driving as some others have done. I hoped that by checking it by the FSM method, it would be enough. It responded quickly to opening the throttle and had the specified pressure.

I have checked timing, it was fine.

I will check the coolant sensor connection and resistance this weekend.


Does anyone know which sensors are still active when the computer goes into open loop? And at what throttle % does that happen at? Above 70% maybe? The reason I ask is because I would think that if the problem is a sensor that gets ignored during open loop, then it would run at full power after a certain throttle position. I cannot tell a difference between 50% throttle and 100% most of the time. This is the only 4.0 I have driven...should I be able to tell? :eyes:

Thanks for any help. I have been chasing this on and off for quite a while now.

What are the values on your tps test?

Honestly, I can't remember. I replaced it a while ago, maybe 8-12 months. I think I will double check it this weekend as well. Perhaps it is defective.

How do you test the coolant temp sensor?

I tested the MAP sensor, TPS sensor, Air intake temp sensor, and the Coolant temp sensor over the weekend.

They all tested good according to my FSM.

So, I am not quite sure where to go from here, except maybe the fuel pump is weak. Pressure tested good per the FSM but I never tested the volume. It is probably the stock pump with 190k+ miles on it.

My other shot in the dark is the muffler. When I got it, the cat was in pieces and although I replaced the cat with a high flow one, I often wonder how much of the old cat is stuck in the stock muffler and causing weird power issues as the pieces move around and block/unblock different parts of the muffler. This is probably a complete stretch, but weirder things have happened. :rof:

bigalpha said:

How do you test the coolant temp sensor?

Click to expand...

You check the resistance of the sensor at different temperatures and compare the values to the FSM. Resistance should go down significantly as the temp goes up.

I suppose my real question to that was what are the values of the resistance reading? Is this for the CTS in the rad or the block?