2 Different Mechanix STUMPED!!!

[OL Crony]

This problem arose a couple of weeks ago and was intermittent, now it seems to be more permanent.

I have no acceleration - LITERALLY - I cannot drive my truck much over an idle. When I step on the gas it must only be very light otherwise if I depress the pedal too much, I get a series of different reactions, but the most common is that it almost dies, give a hurky jerky action and does so until I let up on the pedal. The other day, just out of frustration, I floored the pedal, no difference, except that when I let up on the pedal - backfire. The least common reaction is that I have no acceleration but with out the jerky motion. I could have it floored and cannot get anywhere. During all of this, my check engine lite does not come on. when the truck is in park, i can step on the gas all i want and it works just great, i does this when i want to drive down the street.

The mechanix at two different garages (one of those garages being the local ford dealership ) say that the computer is not reading out any error codes. One mechanic suspected that I got some bad gas but his test said that was not the case. His volume and pressure test also fell within specs. The plugs and wires were checked those passed inspection. I seem to be getting good exhaust - is there a such a thing as an exhaust pressure test, i saw the one mechanic just put his hand back there and rule out a plugged converter. I recently replaced the air filter and disconnected the battery to no avail.

Any ideas that you have would be very appreciated. Idling my truck down the street to work and back is gettin old. Thanks again.

az

 

[nitestalker]

my wifes focus did the same thing and turned out to be the fuel pump. it will read perfect then give it gas and poof there it was.

 

[OL Crony]

i would hate for it to be the pump cuz i had that replace a couple of years ago and it wasn't cheap.

how did you end up determining that it was the pump?

 

[nitestalker]

my friend if a master ford mech.

 

[410Fortune]

I believe your computer has failed.
I have seen several Gen I Explorer 4.0L OHV PCM's go bad.
You have a couple of choices, find somebody with the same truck, and I mean same truck, 91-92 with the same transmission either Ranger or Explorer and try their computer
Or have Ford do a pin out diagnostics on your truck, about $300

You need to get the code off your PCM and find a replacement, they go for about $30-$100 used, at junk yards or places like advanced auto.

I was going to say its a fuel problem or a clogged cat, but you appear to have had those both checked and no CEL and the computer not throwing codes leans me towards a failed PCM.

Like I said this is not all that uncommon and can be very frustrating to diagnose.

I hope when you say pressure test they tested both the vacuum and fuel pressure, correct?
If the fuel pressure is within spec that rules out a failed pressure regulator, pump, and clogged filter.

Also the catalyst inside the cat can come loose and only fall to the back of the exhaust and clog your exhaust when the truck is moving forward, usually you can hear a rattling from under the truck when this happens...

So if the fuel and exhaust systems check out I would move on to checking the PCM or computer itself.

Since this is a A4LD truck and it only runs like crap when you put it in D you may also have transmission problems, either wiring, vacuum, or torque converter, but usually that will throw a CEL and your PCM doesnt have any codes in it.

Keep us posted Hope this helps.

 

[CDW6212R]

Ditto, the pump is my thought. I had a 1985 Lincoln that drove like that when I bought it. The fuel pump was bad, but not completely bad.
The fuel pressure should be checked, at idle, and while giving it throttle, with and without the vacuum line attached.

Another possibility is a vacuum leak in the fuel pressure regulator line. I had the rubber union at the regulator expand from fuel(backfires), and leak enough that it hesitated often while accelerating. The rubber union is only an inch long, and should not be very loose. LOL,
DonW

 

[CDW6212R]

Vacuum line leaks

The 93 Limited that I had this hesitation on backfires very rarely(6 times per year), while starting. For a couple of years I have had to turn the key on and off three times before engaging the starter, to start the engine. It only does it on initial start each day. I have too much leak down of fuel pressure, and I haven't worried about it, because I have been planning to replace the whole drivetrain.

Others with similar problems should check all of the vacuum lines, especially the rubber unions. The rubber will expand, and cause a leak. Good luck,
Don W

 

[Eneurb]

Have you tried it with the MAF sensor disconnected???

 

[OL Crony]

the tests that i did see regarding the pressure test only involved fuel pressure. i am not saying that the mechanic did not test the vacuum but i did not witness that. what i did witness was a gauge that he hooked up to the engine, and it looked like the fuel went into this gauge and gave a reading. he said something like the pressure should be 40-45 in the prime position and 30 to 45 while it was running. my memory could be failing me but that's close to what was said. both of those checks tested within spec. to him, that test ruled out fuel regulator, pump and filter.

 

[OL Crony]

where is the computer and how do i get the number off of it?

i can get these at local auto parts stores?

or only at junk yards and the sort?

 

[410Fortune]

thats right.
You can check for vacuum leaks yourself with a can of starting fluid and the engine running.

I personally think you shoudl try to swap with another trucks computer, 1 bolt, takes about 15 minutes. Disco the battery, swap PCM's, plug imn battery, fire it up. I bet you the truck runs perfect with a working computer.

The fact that you have no codes and this is happening points me to the PCM, L personally have seen this 4 times on 91-94 explorers and have replaced two computers on my truck after a refurbished unit from Ford failed.......

 

[aldive]

What is your fuel pressure?

 

[OL Crony]

again regarding the computer...

my non ford mechanic said that he couldn't get any readings from the computer.

the ford mechanic said that he did get a stored code, but it did not pertain to the symptoms that i was describing. he said that it was a bad solenoid - where? i don't remember. but he specifically said that he tested that solenoid, it was indeed bad, but wouldn't cause the mechanical difficulties that i am describing.

 

[OL Crony]

the fuel pressure when i was looking at the gauge read at 42.

 

[CDW6212R]

My 93 has never thrown any codes. There are very few vacuum lines, and they are basically all easy to check. Before spending any more money, check all of the connections. It's quick and free, LOL.
Don

 

[OL Crony]

regarding the fuel pressure test...

the engine was running, when he disconnected the hose to hook up the gauge the engine quit immediately. he was puzzled by that, he thought that the engine should've kept running a bit longer. but that is the test that he called the prime position, i assumed that the pressure would be different at different points along the key's position in the ingnition switch.

i then do believe that he did turn the truck on - look at the gauge, went into his office and came back out, told me the numbers which i don't remember and proclaimed that they we're in spec.

 

[aldive]

What was the FP before cranking with the key on?

 

[OL Crony]

regarding the computer swap...

i don't know anybody personally that has a similar year explorer. certainly i see them around town, but i don't know those people.

i suppose i could go to the junk yard and pull one of the units but there is no way to tell if those computers are functioning normally.

if i bought one and it didn't solve my problems, i guess i could return it.

 

[OL Crony]

the fuel pressure before cranking with the key on was at 42. the key was in the on position with the engine running, it quite when the mechanic hooked up his gauge, but the key was still in the on position.

 

[410Fortune]

the pressure at idle should be steady around 35# I believe.
The PCM's at the junk yards have usually been tested before they will re-sale them, meaning the engine is trun before they pull the truck apart.
The ley is to match the code on your computer with the one you get, it will read something like COW-1...And yes you can always return it.
Also you could "test drive" a 91-92 Explorer or ranger that is for sale over to your house and do the swap...but you didnt hear that from me