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1990 5.0 Dies when accelerating

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Old 04-08-2018, 06:00 PM
1990dw
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1990 5.0 Dies when accelerating

So, the issue is the truck sputters and dies (also, "bucks" pretty hard) when driving if you give it more than even 1/4 throttle. Also does it at idle if you stab the throttle, but it's worse when the engine is under load.

Now onto a little backstory. I got this truck as part of a trade. I was told that every sensor was replaced, fuel pump, tune up, etc. and the PO couldn't figure out the issue.

So, I'm attempting to narrow it down. And I'm trying to not just throw parts at it, but just by looking, the tune up parts appear to not have actually been replaced, so it's hard not to just replace it all. I just need some help to properly diagnose it.
And I can't even pull the codes because the CEL does not illuminate. I've pulled the cluster out and put a known good bulb in it's place and no change, so that makes me fear the ECC is bad. I've read a lot of other posts, but no one follows up and posts what actually fixed the issue.

So, if any of the gurus could help me out. I'm happy to start where ever you see fit.
 
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Old 04-08-2018, 07:45 PM
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You need to confirm fuel pressure, should be 32-35psi at idle and should increase towards 40psi under load, it should never drop below the idle readings.
If that checks out then make sure the exhaust isn't plugged, it should not sound like you opened a valve on an air tank when the throttle is opened.
Then you are back to potential electronic causes, could be a lot of things but finding out about the health of the PCM would be a good start. There are other ways to read codes including with a test light, if that fails too then you should probably remove the PCM and inspect it for obvious problems(swollen/exploded caps, corrosion, water damage) but also to probe the connector to ensure you have power and ground on the correct pins. You can also check that the PCM is correct for this vehicle based on the calibration code, and also make sure the TFI ignition module on the distributor is grey and not black.
 
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Old 04-08-2018, 07:54 PM
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[double post]
 
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Old 04-09-2018, 09:39 AM
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Well, checked fuel pressure with a gauge. Shows 35 psi at idle, but when you start to accelerate the motor, it stays at 35 psi and starts to drop as you get higher in the revs. I pulled the fuel filter and blew backwards through it, no issues. The fuel pressure regulator looks like it was put on 1 mile ago, so hoping that should be fine. I sprayed it with starting fluid and it doesn't seem to have a vacuum leak. Went ahead and sprayed everything else, and didn't find any other leaks. So, fuel pump then?

I looked at the distributor, and the TFI module is in fact grey. The exhaust doesn't sound like it's blocked, definitely doesn't sound like a sudden release of pressure or anything. (Some one cut the cats and all out of it already.) I didn't pull codes yet, because of the findings.

Very appreciative of the info so far!
 
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Old 04-09-2018, 11:42 AM
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I just got finished trying to pull the codes with a test light. (Well I made a test light with an led headlight, but anyway.)

So, at the ECC Test Port with the key off, I hooked the jumper from pin 2 to the STI and I hooked the negative of the light up to pin 4. Then comes the strange part, which is as soon as I touch the positive of the light to the battery, my light comes on. But at key on, I get no flashes. My test light goes out, and the check engine light on the cluster is just barely illuminated. I assume none of this is normal.
 
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Old 04-09-2018, 07:27 PM
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Surely someone has had this issue before. Bump.
 
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Old 04-09-2018, 08:56 PM
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I would use an incandescent light, not led.

What does the fuel pressure drop to if you drive the truck? Raising the rpm in park (no load) doesn't tell you much, generally. However, the fact it dropped at all during your test suggests the pump is failing or otherwise cannot build pressure. As throttle opening increases, engine vacuum decreases, and fuel pressure regulator should then raise fuel pressure at the rail.
 
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Old 04-10-2018, 05:33 PM
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Tried an incandescent light, and now it doesn't light up at all at key off. Just primes the fuel pump, for some reason. Still no flashes at key on.

Pulled the computer out. Looks like it was bought from a JY or something, because the model and info is written on it in sharpie.

Also, checked timing. (Pulled SPOUT) was at 10 to 11 degrees BTC.
 
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Old 04-10-2018, 08:48 PM
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Do you have 5 volts on ths STI connector ?
 
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Old 04-10-2018, 08:59 PM
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Originally Posted by vjsimone View Post
Do you have 5 volts on ths STI connector ?
I did not know that it was a 5v reference. I will triple check.

I assume that it would since it makes sense to me why the LED light illuminated but the incandescent did not. Since the LED was variable voltage, and the other is 12v.

Thank you vjsimone!
 
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Old 04-10-2018, 09:18 PM
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My truck did the same thing when my fuel pump in the front tank went bad. Does it do it when using both tanks?
 
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Old 04-10-2018, 09:27 PM
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Originally Posted by 1990dw View Post
Pulled the computer out. Looks like it was bought from a JY or something, because the model and info is written on it in sharpie.
Not a good sign, but loads of computers are swapped out as an attempt to solve the issues.
Like Conanski stated, if you can find the Calibration Code, on the valve covers maybe, then you may be able to verify proper computer.
Take some photos of the numbers/bar codes on your computer.
 
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Old 04-10-2018, 09:39 PM
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Originally Posted by 1990dw View Post
So, at the ECC Test Port with the key off, I hooked the jumper from pin 2 to the STI and I hooked the negative of the light up to pin 4
Pin 2 is ground, so connecting the STI to pin 2 grounds the STI 5v to 0v and this initiates the Self-Test sequence.
 
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Old 04-10-2018, 10:02 PM
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Originally Posted by anthonym92 View Post
My truck did the same thing when my fuel pump in the front tank went bad. Does it do it when using both tanks?
My truck only has one tank. I thought it was fuel pump also, but all this no CEL stuff has me worried.
 
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Old 04-10-2018, 10:11 PM
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Originally Posted by vjsimone View Post
Not a good sign, but loads of computers are swapped out as an attempt to solve the issues.
Like Conanski stated, if you can find the Calibration Code, on the valve covers maybe, then you may be able to verify proper computer.
Take some photos of the numbers/bar codes on your computer.


I did find this calibration code in the door jam, but I can't find anything online that makes sense of it.

I should have taken a picture of the label on the PCM when I had it out. Do you know what PCM is suppose to be in this truck?

1990 F150 5.0, 5 speed, 4x4

Originally Posted by vjsimone View Post
Pin 2 is ground, so connecting the STI to pin 2 grounds the STI 5v to 0v and this initiates the Self-Test sequence.
I misunderstood, I will check and verify that STI actually has 5v. Using pin 2 as a ground and the battery as a ground.
 

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