New heads put on, possible O2 sensor, maybe crankshaft position sensor? Help

[Ericalan350]

1994 4.0l OHV, new heads replaced and running, upon test driving the truck it hesitates.

I think (key word there) it may be O2 sensors (less than a year old motorcraft) or possibly a crankshaft position sensor (mine is a couple years old and a little dirty).

Anyone have any ideas? Thanks in advance for any helpful advice

 

[RonD]

Welcome to TRS

Not crank sensor, engine simply will not start or run if crank sensor is bad or intermittent.

O2s are not(can't be) used until they get above 600degF so takes a few minutes after cold start(5 min) for computer to use them.
If hesitation happens just after start up as well as when warmed up then not O2s

Check for vacuum leak, easy test
Warm up the engine until temp gauge is just below 1/2, normal operating temp
Let it idle and unplug the 2 wires on the IAC Valve
IAC Valve will close all the way and engine RPMs should drop to 500 or so, barely running, engine may even stall, either is GOOD, it means no vacuum leaks.

If idle stays high then there is a vacuum leak, maybe a hose got missed or it cracked when doing the engine work.
PCV Valve and brake booster hoses are the first to check

Make sure MAF sensor is clean, needs to be done ever few years<<this causes a known "bogging" issue on the 4.0l engines
Also make sure TPS(throttle position sensor) is hooked back up

 

[Ericalan350]

Welcome to TRS

Not crank sensor, engine simply will not start or run if crank sensor is bad or intermittent.

O2s are not(can't be) used until they get above 600degF so takes a few minutes after cold start(5 min) for computer to use them.
If hesitation happens just after start up as well as when warmed up then not O2s

Check for vacuum leak, easy test
Warm up the engine until temp gauge is just below 1/2, normal operating temp
Let it idle and unplug the 2 wires on the IAC Valve
IAC Valve will close all the way and engine RPMs should drop to 500 or so, barely running, engine may even stall, either is GOOD, it means no vacuum leaks.

If idle stays high then there is a vacuum leak, maybe a hose got missed or it cracked when doing the engine work.
PCV Valve and brake booster hoses are the first to check

Make sure MAF sensor is clean, needs to be done ever few years<<this causes a known "bogging" issue on the 4.0l engines
Also make sure TPS(throttle position sensor) is hooked back up

Thanks for the welcome @RonD

I did read in a post where you did recommend to check the IAC. Question, Do you know where I can find a diagram/layout of the vacuum lines and where they go (or are labeled) so that I can have a check list?

I have cleaned the MAF sensor, throttle body, and replaced the IAC about two months ago with the motorcraft or the closest like part if motorcraft was not available.

Throttle body - just for clarification, hooked back up meaning the pig tail securely plugged in correct?

Thanks for the swift reply!

 

[RonD]

Do the IAC valve test first, that will tell you if there is a vacuum leak

Vacuum lines all connect to the vacuum manifold, on 4.0l OHV, that's located on drivers side at the back of upper intake.
If its not a California vehicle there will only be a few vacuum lines
Power booster
PCV valve
EVAP
Fuel Pressure regulator(FPR, lower intake passenger side front)
Vacuum reservoir(on top of blower housing)

Leaking FPR can cause running issues, check that vacuum line for gasoline, replace FPR if gas found in that line

 

[Ericalan350]

Do the IAC valve test first, that will tell you if there is a vacuum leak

Vacuum lines all connect to the vacuum manifold, on 4.0l OHV, that's located on drivers side at the back of upper intake.
If its not a California vehicle there will only be a few vacuum lines
Power booster
PCV valve
EVAP
Fuel Pressure regulator(FPR, lower intake passenger side front)
Vacuum reservoir(on top of blower housing)

Leaking FPR can cause running issues, check that vacuum line for gasoline, replace FPR if gas found in that line

I will check it out and report back.

I live in Texas so thankfully if it is a vacuum issue, less lines makes me a little more hopeful.

Thanks for the advice man!

 

[Ericalan350]

I started the truck over the weekend, let it get to temp (normal) and pulled the IAC plug. RPM dropped to 500 to 700. Plugged it back in and RPM went back up to 1K.

I also just for grins pulled the plug on the MAF. CES light came on but motor ran the same.

Any thoughts?

 

[AdirondackRanger]

Been sitting for a while? No vacuum leaks. How about fuel condition? Maybe fuel stabilizer and dry gas treatment?

 

[RonD]

I started the truck over the weekend, let it get to temp (normal) and pulled the IAC plug. RPM dropped to 500 to 700. Plugged it back in and RPM went back up to 1K.

I also just for grins pulled the plug on the MAF. CES light came on but motor ran the same.

Any thoughts?

That means the Computer(PCM) is setting idle at 1,000

I assume the 500 to 700rpm was engine struggling to run?

Does the TPS(throttle position sensor) look newer, like it was replaced?
Maybe they didn't adjust it for Closed throttle, so PCM never sets idle RPMs

 

[Ericalan350]

Been sitting for a while? No vacuum leaks. How about fuel condition? Maybe fuel stabilizer and dry gas treatment?

The truck is/has been my daily. The fuel filter is less than 10K miles old (less than a year)

 

[Ericalan350]

That means the Computer(PCM) is setting idle at 1,000

I assume the 500 to 700rpm was engine struggling to run?

Does the TPS(throttle position sensor) look newer, like it was replaced?
Maybe they didn't adjust it for Closed throttle, so PCM never sets idle RPMs

correct, the engine did struggle at that RPM to run. I replaced the tps with the motorcraft part a year ago. I have not adjusted the tps for closed throttle. The ECM was replace a year ago as well.

 

[Ericalan350]

Am I missing anything on the check list besides rechecking the gap on the spark plugs?

 

[Ericalan350]

Any other potential causes or areas to check regarding my issue? I hate to take it to a mechanic and say help.

 

[RonD]

Computer was replaced?

Did the high idle start after that?

 

[Ericalan350]

Computer was replaced?

Did the high idle start after that?

I have not had an issue with high idle ... so far. the truck idles around 700-900ish.

The ECM was replaced a year ago. The issue I am attempting to resolve has to do with as I accelerate or rev the motor up, there is hesitation/stumbling. While driving, it feels/appears to be missing, sputtering, etc. It will rev up but it is not clean or smooth.

I had this issue prior to replacing the head gaskets / cylinder heads. I am at the end of my rope.

 

[RonD]

Sorry I was thinking of another thread.

Hesitation is either lack of fuel or lack of spark timing change

TPS tells computer to add more fuel quickly
It should also retard spark timing(closer to TDC) because the Richer Mix ignites faster than 'normal' air fuel mix.
Thats what vacuum advance did in distributors, when you pressed down on gas pedal vacuum dropped in intake, so advance dropped, spark was retarded for Richer fuel mixes faster burning

You could get a timing light to check if spark timing is changing when engine is REVed quickly

If you slowly accelerate does engine run OK?