Surge/hesitation problem

I'll try to keep it as short as possible and explain what has been done. When accelerating it has a hesitation/stumble. In park if you raise the RPM's, it will not hold steady, fluctuates a couple hundred up and down. No check engine light is triggered.

I picked up a project 2000 3.4 auto with a cracked head. Replaced heads, verified valve lash, checked compression, leak down test, cleaned and flow tested injectors, new timing belt, plugs, wires.

Also cat was Replaced, new O2 and F/A sensor. Ohmed out coils and even swapped with spare coils to test.

Smoke tested for vacuum leaks.

I have a scanner and have overrode multiple controls to verify operations but I am not very familiar with the reading to confirm what is out of range.
Feel free to ask more questions, thanks.

 

Just a guess bit it sounds like a bad TPS. Computer doesn't know where the throttle is exactly.

 

Thanks for the reply but, a sensor from another running vehicle was swapped and the symptoms did not change.

 

Just another guess but it might be intermittently dropping spark. Is the ignition control module working well?

 

It never triggers a misfire code. Interested in what parameters have to be met to trigger code?
I will have to look deeper in the manual about this.

 

What's your fuel pressure? What are your long term and short term fuel trim values? How many g/sec is the MAF sensor reading at idle and 2500 rpm? How about ECT readings through warm-up cycle? What are the upstream and downstream O2 sensors reading? Are you using quality (correct) parts?

 

I have not checked the fuel pressure due to not having the proper adapter here. If I run it up to 5k rpm's or better it doesn't have the feeling of fuel starvation. I know this does prove proper fuel pressure.

I don't have the sensor reading In front of me now, but I will post this information later today.

All sensors weren't OEM but were replaced with better quality parts (Bosch etc.) Also to note this is a California car with one O2 and a F/A sensor.

 

Did did you mean to say doesn't? The only thing that will prove that theory is a fuel pressure reading. If you want to truly eliminate possible causes without costly/timely guessing, then don't skip this. I know it's not easy to check without the proper tools. Your long term fuel trim might give us a clue. The surging sounds like a sticking IAC, but not necessarily the hesitation. Rule out the things that won't require "shotgunning" parts first.

 

No it does go up to 5k rpm's. It is like it will stumble and get past it. It feel like it is low on power.

These are some of the numbers and I will try to attach a screenshot of the short term:
Long FT -1.59
Short FT fluctuate -9.5 to +8.7
MAF idle 4.6
MAF 2500 17.5
ECT 180°
O2S B1 S2 .740

 

That looks like you might have a dirty MAF. Remove it and look at the sensing wires closely. That can easily be cleaned if so. The MAF should be approximately 3.5-4 g/sec idle and about 12.5-13 at 2500. Those short terms fuel trims should be steady and close to zero at idle. Is that graph at idle?

 

I did clean the MAF sensor when the intake was off. Yes that graph was at idle. The long term fuel trim was steady at that value.

Did you mean to say long term fuel trim should be steady and close to zero?

If you need a graph at rpm, let me know.

I will take another look at the MAF sensor to see if something didn't get on it during reassembly.

 

They both should be close to zero, but the short term is what's happening now, the long term is cumulative. Make sure you clean the little sensing wires deep in the cavity. Those are the ones that are going to cause erratic fuel trims, not the big bulb looking thing that's easy to see.

 

I pulled the MAF sensor out of the air box and cleaned, didn't visibly look dirty.
Idle numbers are 4.1 to 4.3.
2500 rpm numbers are 16.7 to 16.2
Fuel trim numbers stayed similar to before.

 

I have an 02 and it had a similar issue twice and it was hard to ID why it was happening. The 1st time it was the throttle body which was replaced. The 2nd time it was the cat. The material inside had broken up some and it was moving around when I drove. All of the material inside the cat was removed and it fixed it. In TX we dont need to pass a tailpipe inspection after a car is 24 years old so mine will not be checked anymore. Removing the material didnt trip any lights on the dash and it really doesnt sound any different.