loss of power when warm

[DFracing]

Hey guys i have a 2002 sohc
had the timing chains replaced and heads rebuilt about 30k miles ago,
ive had a problem ever since
when the motor is cold it has great power, once it comes up to operating temp i have no low end, but once the motor hits 4k rpms it lights up almost like turbo lag and screams. i dont have any check engine lights it seems to be getting enough fuel, no sputtering or anything like that.

any ideas for me? is it possible i have bad cat?

also, i just replaced the spark plugs, wires, cleaned the maf.

 

[alwaysFlOoReD]

I know nothing about sohc motors but that sounds like cam timing. The 429/460 timing chain in the 70's was IIRC retarded by about 8 degrees. If you used a timing set from the earlier motors it woke the motors up a lot. Maybe the rebuilder messed up the timing chains?

Richard

 

[DFracing]

i thought that for a while but i just find it weird that it only does it once the motor is warm which leads me to believe that it is electrical

 

[RonD]

When an engine is cold the computer is running in Open Loop, "choke mode".
It only uses the MAF sensor for fuel mix and runs a rich preset fuel/air mix, it also advances timing and raise idle, just like a choke did on a carb.

After warm up the computer uses direct fuel/air calculations based on MAF data and looks at the O2 sensors to fine tune lean/rich.
This is Closed Loop operation
Might be worth a look on a live scanner to see what fuel trims are doing when warm.


And just as a test for something else I would check the PCV valve hose especially the elbow, these only last about 10 years, and they would cause a lean mix if leaking, and only warm since preset mix is already rich.
The PCV hose is a very common problem on Fords that are getting older

Also, on warmed up engine, unplug the vacuum line on the EGR valve and see if idle changes, it shouldn't, if it does exhaust gas may be fouling mix at lower RPMs.

 

[Caesar]

I was also going to suggest looking at the live data values when cold and warm to see what values are out of place. Take a picture of the values when it is cold and then compare them when it warms up. There could very well be a sensor that is throwing your mixture out of whack.

I recently had an engine coolant temp sensor that was bad and it was causing an extremely lean mix when the truck warmed up. We identified it only after checking the live data and saw the temp value at 400 degrees! The truck would barely start in my case. Swapped the aftermarket sensor for a Motorcraft sensor and it runs perfectly. I am now getting better gas mileage too. It didn't throw a code, it just stalled and ran like crap until we figured it out. I have a 4.0L SOHC from a 99 Explorer in my truck.

 

[DFracing]

hm interesting ill look into that as i remember there are two plugs under the throttle body (for coolant temp and something else im assuming), i could have gotten them mixed up when i put everything back together, seeing as they were both the same style of plug and so close to each other what i have a plug in ultra guage, that i can set up, what values do i need to look at, i can do that tomorrow

 

[Caesar]

You could take a look, but the two sensor connectors are slightly different (each has a small offset tooth) so they can't be mixed up. It will be tough to tell which values are off unless they are way off like mine was. 400 degrees was easy to identify as faulty in my case. But definitely look at the ECT temp sensor data, as it does have quite a large effect on how the engine runs.

With that said I don't necessarily think the ECT sensor is the culprit, but it could very likely be another sensor. I mainly wanted to use the ECT sensor malfunction as an example of the consequences of a bad sensor. Maybe post your live data (cold and warm) here for other members to check out. Another more primitive method is disconnecting any suspect sensor when the truck is warm to reset the value to default and see if the engine behaves differently. Maybe try the MAF sensor and ECT sensor. That can help to point you in the right direction sensor wise. From what I understand bad O2 sensors usually throw a code, but they can contribute to poor performance once the ECU switches to upstream after it warms up.

 

[DFracing]

okay i did a little test here, i had driven the truck about 2.5 hours prior, coolant temp was down around 130 ill post pics and you tell me
PRE DRIVE :

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POST DRIVE:

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[DFracing]

first reading was directly after starting the truck at idle, and the second was after i was done driving while at idle, i know it doesnt tell much, but not sure what these are supposed to read

 

[Caesar]

Well the ECT temp appears to be correct. How is your gas mileage if you don't mind me asking. My truck has plenty of power but I get around 15-16 mpg, and it used to be closer to 12. My father in law owns a shop and has a pro scan tool/computer that can display all of the engine data and I'll take a few pics so u can compare.

 

[RonD]

Bnk 1 O2
Is the passenger side O2 sensor reading, Bnk 2 would be drivers side
O2 sensors need to be at 650degF or above to work so are not "on-line" until they are heated up
These values change quickly, which is good
.4 and .5 are the sweet spot the computer tries to maintain
This is also referred to as STFT(short term fuel trim)

Lng tr1 is LTFT(long term fuel trim)
The closer to 0 the better but when idling it will climb up, 10 is not bad
This is an average of STFT, and of course doesn't start working until O2 sensors do

Tmg adv is spark timing

Eng F
Is engine temp, it must be pretty cold where you are
Your thermostat is a 192degF and that is your temp, in summer it would be around 210, but in cold weather you are running at the minimum temp which means t-stat is working.

 

[DFracing]

dont even really pay attention to it now-a-days because i do 85% city driving. hardly ever get on the freeway. It would be depressing, but id say anywhere between (10 - 14) 3.55 gears and 33 x 12.5 tires.

RonD: today ill switch the guage over to read bank 2, and see if what im getting on that side is consistent with bank 1

also i live in san diego, temp never really drops below 55, outside temp was about 65 when i did the test

 

[DFracing]

ok guys switched the guages over and i found the problem but need help to find whats causin it

start up :

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after warmed up at idle:

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never moves from 1.56 on bank 2 long fuel trim
also: they bounce anywhere from 2.5 to -1.5 or so

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[RonD]

After warm up
Bnk 1 O2 1
Bnk 2 O2 1

Should swing from .2 to .8
Raise RPMs to 2,500 and hold it there then lower to 2,000, hold, then let it idle again. Remember while this is "live" it is not "real time" there is a delay from when you change engine RPM and when you will see the results on the OBD II reader

Lng tr 1 and 2 are usually more helpful, you want these to show close to 0.00, +/- 5.00 is considered acceptable.

 

[DFracing]

lng tr 2 is stuck at 1.56 once it warms up everythimg else is doing what it should