Hi folks, I'm looking for a little troubleshooting help here. I just bought a '95 box truck with the 5.8. It has been running inconsistently since picking it up. It runs well at idle but under a load it shudders and lacks power like it is bogging down then it will suddenly gain power and run smoothly for brief periods. It is difficult to get it above 60 mph...

Here is what I have done... Removed and thoroughly cleaned TB, EGR valve, IAC valve, tested TPS, checked and re-gapped plugs, changed wires, cap and rotor button and verified firing order. I am at a loss. Any help would be appreciated.

 

Did you check the fuel pump pressure?

 

Did you pull the codes? This is ALWAYS the first thing you should do when troubleshooting problems with an EFI vehicle.

 

No code reader. The truck is not registered yet, but I could sneak it over to Autozone tomorrow I suppose.

I plan to check the fuel pressure tomorrow. I had a motorhome with a 460 a couple years ago and it had poor fuel pressure and acted similar. Thank you so much for the responses.

 

No need for a code reader, just find the EEC Test harness below the drivers side hood hinge and put a jumper(paperclip) between the Self_Test_In and Sig_Return wires, turn the key to run and count the flashes on the check engine light. See the link..
Fuel Injection Technical Library » How To Run a Self-Test

 

I checked the fuel pressure today. It sits steady at 33 psi which is good from what I understand.

I was going to pull codes as per your instructions but I think somebody has removed the check engine bulb. I will have to get the codes from the alternate method when it stops raining.

 

It must be different for the commercial chasis or something... I cannot find the diagnostic output on this box truck. I looked at the driver side fender but it isn't there. I can see the TFI module and two relays hanging loose and possibly the ABS test plug (2 pin?). Could it be located somewhere else?

 

OK, I think I found the diagnostic port... Even though it is a 1995, it appears as though my truck is OBD2 (7 pin) How can i pull codes for this?

 

I decided I would just go over to Auto Zone and have them pull the codes... and an interesting thing happened on the way... It is only around the corner from my house, so I thought I would take my chances and scoot over there with no license plate. No surprise, it ran as it had been running on the way over there. At the stop light it seemed to have a cyclical miss. Run smooth, then run rough, run smooth, then run rough. About a 2 seconds of running smooth and 2 seconds of running rough.

I got the to the Auto Zone and they didn't have the 7 pin adapter. Ugh! So I jumped back in the truck, started it up and it ran like a dream! Totally smooth all the way home. I was so amazed that I decided to sneak out on to some back roads and drive it some more. 25 minutes later it was still running smooth. Forward, backward, under load, idling, just perfect! The only thing I do notice is that the idle is a little high. Even when it was running poorly the idle seemed to be where it should be.

So now I ask... What's the deal? What would cause it to run rought then smooth out completely? I know when I pulled the MAP sensor off it idled up, but it still seemed to have the miss. I am really at a loss. I am grateful to hear it running like it should but I am hesitant to claim a victory just yet, especially now knowing what changed.

 

If it sat for any length of time before you bought it, I'd say that was the problem. As for the shudder, if it's doing that on the highway and it sounds like you're riding on the rumble strip, that's the transmission torque converter lockup solenoid sticking. Pour a bottle of Lucas Transmission treatment in the transmission and that'll cure it for a good while. If and when it happens again, pour another bottle in.

 

No, it wasn't the TC lockup. It did it as much or more often at very low speeds. on takeoff, reverse. It's hard to say what it was, but I am thinking MAP sensor. I wonder if something I did actually did help and it just took a little driving to show. Does that make any sense?

 

Injectors could have some trash in them? You can do a DIY injector cleaner kit from Autozone or have it done at a shop.

Nate

 

It's not that hard to do a "real" cleaning on fuel injectors. I've done it on a couple of 302 v8s.

Start by removing the upper intake manifold, and then remove the injectors with the fuel rails. Then I used a 12v battery and alligator clips to activate each injector while shooting something like Gumout into it from the squirter end. I put the tip of the straw right up against the tip of the injector, and gave it a good douching to clean out any built-up gum and flush out any particles. That worked wonders on my 25 year old Mustang.

On my friend's engine, we found a couple of injectors that did not want to open up with the applied 12v (no wonder it was running like it was on 3 cylinders!). We soaked their tips in Berryman's B12, and managed to persuade one of them to open up by pushing on the pintle with a plastic rod. The other one was just stuck, so we had to replace it with a similar one we got from the junkyard. That engine runs real smooth now as well.

 

Wouldn't a bad injector throw a trouble code?

I did finally pull codes and the only one it showed was a bad EGR position sensor which I replaced. It made no difference.

Would a plugged fuel injector suddenly clear up when the engine gets warm?

 

Smoothing out when it warms up? That's starting to sound less like the fuel injector. How do the spark plugs look?

I guess if you run a KOER test, it could detect a bad injector; it may show up as a consistent lean condition. But I never got a code for that either when mine got clogged up. I got a very consistent miss in my engine, and it went away after I cleaned out the injectors. I also get the EVP error codes all the time, but the EGR is working fine.