Howdy all.

Been having an issue with my 98 XLT. There seems to be a very small sweet spot in the throttle where the truck will run quite well, its maybe 1/4 throttle.

Anything over that seems more noise than go and I start to get a bit of pre detonation coming from the left bank, at least thats what it sounds like.

Any idea's?

 

Partially plugged exhaust?

 

Had not thought of that. Not sure how I would go about checking that. It sounds ok at the tailpipe and I don't think I would be too popular to run it open exhaust to try to diagnose a plugged exhaust,heh.

 

I agree.That sounds very similar to how my work truck ran when the catalysts were melted.
JL

 

Thanks. Will do what I can to troubleshoot the exhaust system.

 

Just put her up on the ramps to have a look at things. I can not figure a reliable way to check the cats. I banged on them and they don't seem to be rattling. One thing I did notice was that after just the short run time moving my truck around the yard and on to the ramps the drivers side front cat is substantially hotter than the passenger side. Could this be a bad upstream 02 sensor? Maybe time to find a code reader.

 

The cats on my work truck were melted,and would not rattle,so that's not a real good indicator.
JL

 

If cat is bad will it run hotter than normal?

 

Definitely.
JL

 

Somewhere I read about a spec/procedure that utilizes the differential exhaust pressure sensor lines to gauge exhaust backpressure in order to evaluate CC &/or "plugged exhaust". There was a lot of resistance to the idea these motors sensed exhaust backpressure.

As far as it running "hotter than normal" its been claimed by several members that FORD sought to reduce customer complaints by controlling the temp gauge by the computer, so as to eliminate many of the normal ups & downs, keeping the needle close to center, unless an unusual situation persists. As a result it might be difficult to see any minor "hotter than normal" change.

 

Sorry I think I asked the question wrong. I was referring to the temp of the cats not the engine.

I had the local napa pull codes today and they found no faults with their fancy snap-on tester.

I have had several people today tell me to ignore the spark knock in the 4.6, saying "thats just how they are". What???

 

good grief, what is wrong with you guys, its like every time someone complains of a loss of power, you guys seem to always jump right to clogged cats. Lets take a more realistic and purposeful approach. There are a variety of sensors that tell the vehicle information about how much air is getting to the engine, how hot and dense that air is, how much throttle you are applying, etc. Using this information, combined with a built in fuel table that assumes a certain fuel pressure and injector flow rate, it calculates how much fuel the engine needs with amazing precision.

However the failure of one or more sensors will result in inaccurate calculations, which can cause poor performance, loss of throttle response, pinging, poor gas mileage, etc. Your symptoms more closely match the symptoms of either a low fuel pressure problem, indicating a plugged fuel filter or weak fuel pump, a lean condition, a dirty MAF element, or a host of other problems with fuel delivery or bad information about how much air is flowing into the engine. Allow me to explain.

If you fuel filter is plugged, or an injector is restricted, or the fuel pump is weak of isn't flowing properly, or the fuel pressure regulator is regulating too low, then the computers fuel tables are wrong. They tell the computer to open the injectors for x amount of time, and it expects y amount of fuel to flow. When any of these conditions is present, the amount of fuel that goes to the engine is too little, which causes a loss of power and pinging. This condition can be verified by testing the fuel system.

When the MAF gets dirty it tends to report to the computer that x amount of air is flowing into the engine, so the computer determines that y amount of fuel is required, which is less than the engine actually needs. The result is loss of power, pinging, and decreased throttle response. This condition can be verified by confirming that the fuel pressure is within specs and by checking the Long Term Fuel Trim with a scanner (not a code reader). A positive fuel trim can possibly indicate the MAF. The MAF can be isolated by cleaning it (which should result in an improvement if the element was dirty), or by replacing it (make sure other possible culprits are isolated first). A vacuum leak will have a similar effect, though vacuum leaks tend to become less noticeable as throttle increases.

Other problems that can cause pinging are a bad ACT sensor (Air Charge Temperature sensor tells the computer the temperature of the incoming air at the manifold), too low a fuel octane. Use the fuel recommended in the owners manual, should be 89 or 91, but don't take my word for it. I have heard of bad O2 sensors causing you type of problem, but that is not very common, they usually cause a rich condition as they age. Defective ERG components can cause a lot of strange results, and can include pinging and a loss of power, if you truck has an EGR, make sure it is working properly.

As for plugged cats. The cats do not plug themselves. Usually another problem causes them to fail. I'm not convinced your cats are plugged, because the increase in backpressure would reduce the combustion efficiency, which would prevent pinging, and would tend to make the truck almost undriveable. The fact that you see a temperature rise across them and that you have no P0420 or P0430 code seems to indicate that the cats are not damaged and are functioning as normal as they can under the circumstances. I suggest finding someone with a scanner to retrieve the Long Term Fuel Trim and post the values here.

 

Wow, that's a ton of info there Bear River, thanks!

I was starting to wonder about possibly bad injectors earlier today. Will get it tested tomorrow for the long term fuel trim.

I am also replacing a couple of iffy vacuum lines. I took the egr off today and its clean and functions smoothly so I am thinking its ok. Gotta clean the throttle body too as its really gunked up with sludge.

Again,thanks a ton for expanding on this. Hopefully it will help me get the 4.6 purring again.

 

Ok, did some work today and had the fuel trim test done.

As mentioned before the throttle body was a nasty mess so I took it off and cleaned it. During this process I found that the first egr port was totally plugged and may have been plugged enough that it was cutting off the second port. I cleaned out the IAC and also cleaned the MAF.

This seemed to improve things some. I think the spark knock may have reduced a bit but not totally. Was hard to judge as I did not get a very long drive to critique the performance really well.

Also, there was a symptom present before which is still present that I forgot to mention, sorry. Under heavy acceleration I get some mild bucking. Not rip the motor loose bucking but its definitely noticeable.

Ok, so on to the results. I should start by saying that the person who helped me was just learning the scanner and I have no previous experience with them. We did find the readings for the fuel trim but found you could only get actual readings with the motor running. Hopefully this is expected and we did the right thing.

While all may not be needed I will list all the ones we found.

LT FTRM 1: -2.3
ST FTRM 1: 0 to -6
LT FTRM 2: -2.3
ST FTRM 2: 0 to -3.9
ST FTRM 11: 0.8
ST FTRM 21: 0 to -3.9

 

Finally got this figured out last night. Taking my dad's advice I warmed the truck up after dark last night and popped the hood. I was treated to quite a light show. Arcing all over the place on the plug wires.
I yanked em out and took them back to NAPA. They were the cheap 5 year wires but still under warranty. I upgraded to the Belden Max wires and she is running tops again. No more lean miss and no more surging at high throttle.

I know motorcraft parts are preached around these parts and I would use them if I had more money. My upgrade to premium lifetime wires set me back $36 where motorcraft from the stealership would have been $160. I do plan to replace my autolites with motorcraft asap as I have found the exact part number at Auto Zone for $4.99 per plug VS $8.92 at the dealer.

Thanks again to those who offered help, it was and is very much appreciated.