Stumbling under load or at high rpm's no load

tractor_boy · Apr 27, 2009

Hey guys I'm getting pretty frustrated with my truck. It's a 93 4.0 with 140,000 miles.
I just did a complete tuneup at 110,000 and now it's acting up. When I am accelerating the engine seems to misfire. It's not a real consistent misfire, and happens on average it feels like twice per second. The engine temperature, rpm, gear I'm in, or throttle position seem to make no difference at all. No pinging or knocking noise, just the stumbling symptom. It feels almost as if the clutch is just slipping for a bit and then quickly grabbing again. I know that isn't the case (it had a new Valeo clutch 20,000 miles ago), but it might help to describe what's going on.
So I pulled all the spark plugs and they are all very clean and good condition and very little variation in appearance from cylinder to cylinder. I checked all the wires with my ohmmeter and they're good. Thought maybe the coil pack might be getting weak so I swapped it off my 94 4.0 ranger, no difference.
Changed the air filter, cleaned the MAF, no difference.
Fuel injectors were rebuilt at 120,000. I changed fuel filter and checked fuel pressure afterward. It was roughly 5 psi below spec both at idle and at full throttle, so I dropped the tank and put a new high flow pump assembly in. Absolutely no difference as far as the stumbling, but my fuel pressure is back up where it should be now.
Then I was driving today and the check engine light would come on at random times, and then finally stayed on. Shut the key off and the light is gone when you turn it back on.
I plugged it into the $3000 genesys scan tool at the shop where I work and found no background computer codes.
Then I put the tool in datastream mode to look at all the sensor values and noticed a long-term fuel trim of -6% on bank 1, 0% on bank 2. All other values were well within spec and seemed very reasonable. Figured that might mean a bad 02 sensor, so I bought the sensors and replaced tonight. NO DIFFERENCE!!!
I am tearing my hair out and am becoming ashamed that I call myself a mechanic at a shop whose specialty is none other than driveability and diagnosis- but at least it's my own vehicle and not another customer's. :bawling:

Please help me guys if anyone has had similar problems or has any ideas, I'm willing to try pretty much anything at this point!
Thanks
Chris

rangerty · Apr 27, 2009

wow bud thats heard core... i really do wish i could say that i have words of wisdom but you just said all of what is going on with my truck right now and just had it fully rebuilt new everything and then all this started up... =-/ i was going to try and take it into a shop to see if they might be able to find out whats going on but if you have all the tools and cant get it i dont know if i wanna pay the $85 to get looked at... if can find out what is going on with mine you will be the first to know... but if you get yours going post that bad boy up..

-Ty

tractor_boy · Apr 27, 2009

I might go try swapping crank position sensors between my trucks... Just a thought, and also another distraction from my studies lol. :yahoo:

Well it's good to hear others have the same problems haha. I rebuilt my engine transmission t-case, and suspension only 20,000 miles ago too.
If I can't get this figured out in the next two weeks I am giving up on it. Not just from frustration, but also because I am pulling the 4.0 and swapping a cummins turbo diesel in as soon as school is done. I have it sittiing on a stand in the shop right now. That just might solve this problem, even if in a very roundabout way. :icon_rofl:

I just don't want to destroy the engine or be stranded on the road before this two weeks is up.
Chris

H munster · Apr 27, 2009

Not saying its your problem but I have had plug wires ohm good but still be bad. And i have had plugs look fine but also be bad. Have you checked your grounds? Bad ground wires will do lots of crazy stuff.

rangerty · Apr 28, 2009

i wish i had an option like that but this is my only way around now that i got rid of my car.. to bad i did know my truck was going to do this before i got rid of it.. =-/

H munster said:
And i have had plugs look fine but also be bad. Have you checked your grounds? Bad ground wires will do lots of crazy stuff.

ya they are solid from what i can see...

nitrofan1 · Apr 28, 2009

If the codes were not there when you used the scanner you may have a bad PCM. The codes should stay in memory for 40 drive cycles unless they are manually cleared. If the light was on yet no codes found and you didn't remove the negative cable before reading for codes you may have a bad computer. This can also cause your skipping while under load. The only way I know to check an OBD I computer is though a shop that has equipment to detect a bad unit. The hard part is finding a shop that had the ability to do it.

tractor_boy · Apr 28, 2009

You're saying that even when the CEL just barely flashes on for a second, it's still gonna leave its trace in the computer? Makes sense, and my truck is actually the first one I've noticed that doesn't leave a code like this.
Chris

rangerty · Apr 28, 2009

small update.. I finally made it out to get a reader.. when i put it all in and read it out it is coming back with a "random miss fire" the codes are being pulled from cylender 5 and 6... now i just have to try and find out why..

Reading in this little book it was saying that a flashing check engine litecould mean something a bit more. If that happens the miss fire may couse damage to the cat..

nitrofan1 · Apr 28, 2009

Get those all the time at work. Random miss-fire is usually a bad plug wire. Yes I know you replaced them but even the best brand wires can be bad. lol If they are not that old the parts store should replace them under warranty. Get OE Motorcraft or even go with JBA or MSD wires. REMEMBER!!! You get what you pay for!

tractor_boy · Apr 28, 2009

Well I picked up some new plugs and wires today. Gonna change them for the heck of it in a couple hours and I'll keep you guys posted on the results. Did some more testing this afternoon, driving with the genesys scanner in datastream mode, and with a little propane tank sitting beside me. Conclusion- it's highly unlikely it could be fuel related...
Chris

rangerty · Apr 28, 2009

ya i have gotten bad plugs and wires before... i just got new plugs and wires then swiched the wires back to the old ones because they were better.. found that two of the plugs were sparking very little even though they were new so again i went to older but better plugs.. much better spark.. after all that still no change..

i have been toled it could be this and that so i have changed the EGR valve and start coil.. still nothing..

some one said to check out the crank angle sensor but i dont have the tool.. looks like its going to the shop..

tractor_boy · Apr 28, 2009

IT'S FIXED!!!!
So here's my take on what was going on... Very odd symptoms indeed but now it all fits together and makes sense to me.
I swapped the new plug wires in one at a time, test driving in between, and the problem disappeared after changing the number 6 wire. I guess a wire with 20k miles on it WAS bad after all.
As far as the misfire only showing up with the engine lightly loaded, I guess a small amount of torque on the engine was causing the wire to shift slightly and contact something, and thus ground itself out. Now that I think, maybe I could have reproduced the misfire with the engine at an idle by prying the engine on the motor mounts, simulating a load? Who knows though- I'm just glad I got it fixed and not overly willing to try and get it to misfire once again in order to prove my theory.
Thanks a lot for the help guys... you were on the right track with your advice. I shoulda asked you sooner though and spent only $20 rather than $200. :annoyed:

Chris

rangerty · Apr 29, 2009

right on!! glad to hear. I wish mine was that but i have had 2 now sets of plugs and wires.. what are the chances they are all bad.. =-/

the serch goes on........

greaseyfingers · Jun 11, 2010

tractor_boy said:
Hey guys I'm getting pretty frustrated with my truck. It's a 93 4.0 with 140,000 miles.
I just did a complete tuneup at 110,000 and now it's acting up. When I am accelerating the engine seems to misfire. It's not a real consistent misfire, and happens on average it feels like twice per second. The engine temperature, rpm, gear I'm in, or throttle position seem to make no difference at all. No pinging or knocking noise, just the stumbling symptom. It feels almost as if the clutch is just slipping for a bit and then quickly grabbing again. I know that isn't the case (it had a new Valeo clutch 20,000 miles ago), but it might help to describe what's going on.
So I pulled all the spark plugs and they are all very clean and good condition and very little variation in appearance from cylinder to cylinder. I checked all the wires with my ohmmeter and they're good. Thought maybe the coil pack might be getting weak so I swapped it off my 94 4.0 ranger, no difference.
Changed the air filter, cleaned the MAF, no difference.
Fuel injectors were rebuilt at 120,000. I changed fuel filter and checked fuel pressure afterward. It was roughly 5 psi below spec both at idle and at full throttle, so I dropped the tank and put a new high flow pump assembly in. Absolutely no difference as far as the stumbling, but my fuel pressure is back up where it should be now.
Then I was driving today and the check engine light would come on at random times, and then finally stayed on. Shut the key off and the light is gone when you turn it back on.
I plugged it into the $3000 genesys scan tool at the shop where I work and found no background computer codes.
Then I put the tool in datastream mode to look at all the sensor values and noticed a long-term fuel trim of -6% on bank 1, 0% on bank 2. All other values were well within spec and seemed very reasonable. Figured that might mean a bad 02 sensor, so I bought the sensors and replaced tonight. NO DIFFERENCE!!!
I am tearing my hair out and am becoming ashamed that I call myself a mechanic at a shop whose specialty is none other than driveability and diagnosis- but at least it's my own vehicle and not another customer's.
Please help me guys if anyone has had similar problems or has any ideas, I'm willing to try pretty much anything at this point!
Thanks
Chris

You mentioned that you checked all the spark plug wires with your ohmmeter and they're good.

How many ohms is each spark plug wire suppose to have per foot of wire, as I'm thinking about checking mine, too.

tractor_boy · Jun 14, 2010

greaseyfingers said:
You mentioned that you checked all the spark plug wires with your ohmmeter and they're good.

How many ohms is each spark plug wire suppose to have per foot of wire, as I'm thinking about checking mine, too.

I would consider anything between 10 and 20 k-ohms per foot good.

Haha I forgot about these troubles I was having with the gas engine. I ended up converting the truck to diesel, and it's been running since January with the Cummins in it. No more stupid spark plugs, and no more crappy mileage or low power. :yahoo:

But fixing your spark plugs is definitely the cheaper and faster solution, let me tell you.

Stumbling under load or at high rpm's no load

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