1998 4.6, manual, K&A air filter.

I appreciate any thoughts on this problem.

When I start my truck from completely cold, it accelerates normally until at operating temp.
After the truck warms up it will bogg and act like its fuel starved, missing or detonation as the engine is rough under normal and heavy accleration.
It idles fine, and will seem to accelerate somewhat normally with an extremely light pedal.
It has not thrown any codes... I have scanned for them.
I have cleaned the throttle body, EGR ports, AIC, MAF.
No Joy.

Any ideas?

Thanks for you time and expertise.

 

When was the last time the fuel filter was replaced?

 

I replaced it about 4 months ago.

The truck runs well when cold with deep foot acceleration,
so actual fuel delivery seems not to be the issue.

The problems increase with the system temperature.

 

So it runs fine in Open Loop as evidenced by the cold temp operation. When it switches to closed loop it will use the MAF to determine A/F ratios. It could also be the O2 sensors, are they the original? How many miles do you have on the plugs? Plug wires? They will only aggrivate the problem.

Regards

Jean Marc Chartier

 

Thanks for your reply.

If they were problematic, wouldn't the O2's or the MAF be throwing codes?

plugs and wires and O2's age are unknown as I am not the original owner.
I would default to original, with 67k miles on 'em.

 

You'd think so, but I seam to recall another having problems wich turned out to be o2 sensors that didn't throw codes

 

JMC, when I floor it (WOT) doesn't the PCM go back into open loop mode?

When I tried flooring it, the problem was just as bad or even worse.

 

After your last response I would say check the TPS. It would be possible for it to stick at a valid signal. As you push on the pedal the fuel mixture will not change with incoming air. The truck will also never know it is at wide open throttle therefor causing it not to go into openloop operation.

 

loudist,

There are built in delays before the CPM allows open loop operation. You could in effect never go back into open loop once the engine has gone to closed loop operation. FYI my truck had a 9 second delay which I had changed to 2 seconds.

JMC

 

Well it finally threw a code... #4 misfire... (like I didn't already know it was misfireing).

It seems all the plug and wire advice might be spot on.

Stay tuned...

 

Check your heater hose above #4 spark plug they are notorious for leaking coolant onto the plug wire.

 

Here is the update I promised.
(BTW I am disapointed that some people ask for advice, then never post the resolution to their problem... just a pet peeve of mine).

I changed my plugs, and couldn't have done it without all the 'tricks and techniques' that has been posted on this site...
Thanks go out to all of you.
Plugs were the culprit. The wires that were on the truck were NOT original and are what seems to be a good quality aftermarket 8mm pro fit set with really good connectors, so I passed on new wires and I am glad I did because they weren't the problem...

It seems that the plugs were shorting out...

How you may ask?
How about 1/4 to a 1/2 inch of oil/water in bank 1 plug wells 2, 3, and 4. The plugs had dark discoloration around the bottom of the porcelain and were oily upon removal. Even the socket was oily around the bottom of it. Unfortunatly, that oil funk went down into the cylinder when the plugs were pulled... c'est la vie, not much I could do about it.
On the other bank, there were also dark rings at the bottom of the porcelain on 2 of the plugs, but no oil.
Theory: water from someone pressure cleaning the engine forced into all the plug wells, the oil in bank 1... from the previous owner filling without a funnel and letting it run all down the heads into the wells.

The plugs that were in there were autolite double platinums, I put in motorcraft single plats.
From what I have read the double plats are needed because of how the different banks fire... one bank fires from the electrode to the ground and the other bank fires from the ground to the electrode. The single plats will wear out on one bank in about 20,000 miles or so because of the arc target isn't platinum. Thats allright... in about 6 months, (after the physical trauma subsides from the hand size reduction surgery I am considering) I'm going to try side gapping with some different plugs that I read about on this site.

Summary:
Truck running like a champ! and all the throttlebody, EGR, DPFE, (and whatever else got cleaned) was wax on, wax off for me while good for the truck.

Will let you know about gas mileage soon...
that is, if I can keep my foot out of it...

 

Ok, MPG went from (on average) 12.9 to 15.9 on Texaco 87 octane.
Thats a 23% improvement!

Its almost a wash financially with gas prices increases though, not counting the cost of the plugs.

 

Thanks for the followup!

Glad to here the truck is running good.