Hello ,
I have a '89 F150 Lariat XLT 2WD 5spd 5.0L
I am the same one that had the issues before and now I am having another that is a real pain , I was hoping that you may be able to help me with some ideas about where to go next .
The Truck sat for many years ( 3-5 ) , just had the motor rebuilt .
All controls are new with exception of the Coolant Temp Sensor , The Map Sensor and the Coil and Distributor / Ign Module
( TPS , EGR , EGR Vacuum Switch , 8mm Spiral Ign Wires , FPR , Fuel Pump , Filter , Both Pumps in Tanks were done before I got Truck , have Verified That , all this is new )
Coil is aftermarket that was put on truck about 2 years ago , has considerable rust on it but no sign of leakage , tracing or arcing .
I have run the Fuel Pressure Gauge on it while driving and when any of these issues happen , Fuel Pressure remains the same , about 33 psi , and it does increase with throttle .
My main issue which may be tied to the above is sometimes when you give it just a tad of throttle going to pull off , or just idling , the rpm will drop and the motor will of course stumble a bit , after that if you continue to pull off it , the motor is weak , like it is just waning and then at one point it will start to pull as normal , it will also lightly pop thru intake occasionally , not violet at all , just a tiny pop , this will come and go , the popping is not very often , in 300 miles it may have done it 4 times .
In most situations I would tend to blame the TPS , and yes I know I could have gotten a bad one , but this was present before I replaced TPS and after still present , though the reason I replaced it was because the idle was always a different rpm when you stopped , and that problem was solved when I replaced it .
Also I have run the Fuel Pressure Test Gauge on it on the road and the fuel pressure is maintaining when this issue arises , it is not constant , this is what is so confusing , motor runs beautiful and very strong most of the time , there is no rhyme or reason.

Also it will do the same regardless of the tank you use , I check each tank with on road fuel pressure test and all was consistent and normal.

I am leaning again toward the ECM or the Ign Module , was considering just replacing module with the MSD replacement just to see , it is seeming to me it is could be a timing advance failure , which could be either ECM or Module from what I have read .

The only other variable would be the coil , it is rusty but was replaced 2 years ago before I got truck , but seeing that the motor runs most of the time flawlessly I do think it is the Coil .

The injectors are original as well but again I find it hard to believe all would be failing off idle at the same time .
Please give me the benefit of your experience and tell me in light of all this what you may think it is .


Thankyou Very Much ,

Guy

 

Have you checked for codes again? What brand TPS did you install? Have you measured the TPS output to see if there is a dip/dead spot or non-linear response?

Does the truck exhibit a high idle speed? The statement of a "pop in the intake" leads me to believe the truck is running lean.

 

Pull the codes to see if anything is flagged. A stumble at throttle tip in could also be a vacuum leak, have you measured vacuum or inspected the vacuum resovoirs for leaks? The juice can on the passenger inner fender often rusts out on the bottom or you could have cracks in the vacuum harness that runs around the back of the motor and never see it because it's loomed and taped up.

 

No Codes , all Vacuum lines are like new , I cannot tell you the Vacuum Level as I do not have a Vacuum gauge , guess it is time to get one , I will check the Vacuum Reservoir tomorrow , motors idles steady and smooth , TPS is a BWD , I just put on and no I have not checked voltages , this is a very random thing that happens , outside of this happening about once every 5-10 minutes the truck runs well and has lots of power , the way it feels when it does it and I am accelerating is more like a timing problem , I will go over the vacuum lines again just to be sure , but they are spotless clean and still have their flexibility and good color .
I would think if it was a Vacuum leak it would be consistent and idle would be higher and float a bit , Most of the time I can pull off with no issue and the power and torque are amazing in my opinion , it is just every now and then it does it , for some reason the A/C running will make it appear far more often but then again even with A/C on most times no issue .
What should the Truck Idle at ? Right now after 300 miles , the idle is steady pretty much at 750 , it was staying more at 800 in the beginning , could it possibly be the throttle plate is shutting completely ? And the Map Sensor is original , could that possibly be causing this ?
1/2 to WOT the truck is very strong , no misses or anything .
And more often than not is it like that from idle up , this just keeps happening intermittently , I have read that the modules can have some ghost issues , but do not know if that could cause this , I would imagine it could , and without codes I have no clue , my last issue was the EGR , replaced that and the vacuum switch for it and that fixed that part , as did the TPS replacement fixed the different Idle speeds I was encountering every time I stopped at a light .
I will call rebuilder tomorrow and make sure the TPS was taken off for cleaning the throttle body but I am sure he did not leave it on .
This truck has sort of had this issue since before the rebuild as well , though it was not as bad with fresh motor , this is why I am thinking the issue was pre-existing and maybe in some way was hidden by the bad egr valve and or the motor being extremely fresh , this is why I am so focused on the original parts , the guy that did the motor has a very good reputation , he checks all these things like the vacuum lines and such and replaced many including all hose and serpentine belts , I have been over motor with fine tooth comb , so I assume it has to be a control problem .
If you could provide me with the TPS Voltages , I will check them tomorrow along with the Vacuum canister , I have also separated the 2 & 4 and 7 & 8 plug wires as there is a bulletin stating that this could cause issue by inducing crossfire , I did this when I put the new wires on.
I also noticed tonight as it did it twice going to pick up my son , a 1 hour drive round trip . It was raining and when it did it once pulling off from a stop , once it recovered it broke traction hard , so it seems like it may be a here one minute and then gone the very next kind of thing which is why I kinda lean more toward an electronic early death symptom lol .
I know this is a sad question , but where is the ECM on this model , I have checked under the hood and the passenger kick panel , it was not in either place , have not checked drivers side kick panel yet or under dash , I was thinking it could possibly be a dry connection and wanted to apply some di-electric grease to harness plug , also could you recommend what service manual would be best for me to buy , traditionally I have stuck with Haynes .

 

Alright, my turn.

First off, Yes, Haynes is a great repair manual you SHOULD have.

Second, The ECM, EEC, PCM, Computer or whatever term you'd like to use, is located in the firewall right under the brake booster.
It's far easier to remove it from under the hood than to try and remove it from the interior drivers side kick panel. Been there, tried that, total frustration.

Third, click on this link and do some reading:A9x ECM's (and same years ECM's) Failures Due to Age
My computer was "failing" and I was able to do the repair(s) for about $ 5.00.
A "failing computer" may not display or even store any error codes.
Mine didn't.

I'm not saying this IS IN FACT what you've got going on, but it sure would answer to all of the symptoms you posted above.

Let us know what you find.

Good Luck,

Bob

 

Please define "No Codes". At the very least a System Pass code of 11/11 will be displayed for the KOEO tests and once again for the Continuous Memory (CM) display. Anything less than that means your test results are invalid.

TPS info: Fuel Injection Technical Library » Throttle Position Sensor (TPS)

Many folks have found aftermarket sensor quality to be lacking. It would not surprise me if your BWD TPS is the culprit. Before spending more money check the voltage output with a meter.

T-bob is correct, the PCM on your truck is located behind the driver side kick panel. The PCM is removed from inside the cab for your model year (up to 1991 actually).

For reference:

 

I do not get the 11/11 because it is giving me other codes ( Apply to Cars) in the readout , but there are no codes regarding the truck at all .
I have not run the CM Test , will double check that and get back to you , Thx for the TPS info , I will definitely check that today .

 

Thankyou , I will check out this link and verify / find PCM , I have had tests fail repeatedly checking codes also , where the scanner said it could not communicate and or would not finish test , this is something I forgot to mention .
Thankyou for the info and the link , I will check all of this out , I have a extensive background in electronics regarding repair and service as well as communications / transmitters / Amplifiers , I am no stranger to electronic repairs or electro-mechanical repairs so if this is possible and is problem I will be sure to tackle it , Thanks for the Link , I will surely check into it today .

 

Just took a look at ECM , cannot believe I did not see that !
I am curious what you had to do to get it out , did you have to remove the plastic fender well or will it tilt and slide out of there ?

Guy

 

Oops,

I do yield to your knowledge and experience.

Bob

 

I am not sure where you are getting your code information. There are codes common to both cars and trucks. Please list the codes, and where they were displayed (KOEO or CM), so we can help you diagnose this problem. There is no reason to perform the Key On Engine Running (KOER) test at this point.

If there are any codes in Continuous Memory (CM) they will be displayed after the Key On Engine Off (KOEO) tests are completed/codes displayed. IF there are no CM codes then you will get the System Pass code of 11/11.

If you are having difficulty getting your code reader to work try cleaning the contacts on the Data Link plug. Some contact cleaner may help or in some cases a little scrubbing with a small screwdriver/nail file has proven to be a winner.

 

I was corrected before on this subject. I never owned a 1980-1991 F-series before. I had two 1979s then skipped ahead to a 1993 Lightning (actually two of those as well) then my last 1993 F350.

I also assumed the PCM came out from the firewall side like the 1992-1997 trucks. I was sadly mistaken and summarily flogged in a public forum

 

What rla2005 said, I'm certain he IS correct.

Bob

 

I've experienced the same problem with a customers vehicle and eventually what I found was a leaking 10uf capacitor in the ECM. The vehicle sat for about 10 years and was running fine when parked.All the usual stuff was replaced before the ECM was checked.

 

Post#2 in this thread answers the "how to remove question": https://www.ford-trucks.com/forums/1...lp-needed.html