I have been chasing a funky problem for awhile now. Basically, when I get above half throttle my truck starts to pop like its misfiring or going lean. Now, if I really get on it in first gear it does it the most. If I am only going 30mph and in 3rd gear, and I floor it, it doesn't pop, but doesn't have much pep (partly due to the low speed and 1:1 ratio of the gear). It seems the higher the RPM or more the load, the more the popping. This makes me think its going lean.

My combo is this: 350 with a TPI. Custom chip (via me), painless harness. Th-350 trans, 4.10 gears. If you request any more info I'll be happy to provide.

As I work for a parts house (at least when I am not in school), I can replace things cheaply if not for free. Before I began parts replacement I checked the basics like base timing (set at 6 degrees) and fuel pressure (currently at 40PSI).

Over time, pretty much EVERYTHING has been replaced. When it first began doing this I replaced the fuel pump and filter. No change. I then checked and replaced the fuel pressure regulator diaphram. No change. At that point I though that it might be going lean due to inaccurate air readings. I swapped in a friends good MAF and there was no change.

My spark appeared to be a bit weak so I put in a new coil and module and a wireset and while the spark looked a bit more powerful, the same problem was occuring.

At that point I decided to toss in a new computer and MAF power relay.. After that the truck ran GREAT! So I drove it down to school and within a few days it began acting up again. As it was free, I tossed in a new ECU and there was no change. As it was seeming to act up at a certain throttle point, I though my TPS might have a flat spot. I couldn't see one through my scanner but I decided to put in a new TPS anyways. After that, the truck ran GREAT!

Two days later, same problem occuring. I really have no idea what it could be. It seems to get better after random parts replacement. I also changed out the oxygen sensor as it was time. The idle smoothed a bit but the main problem persisted.

I am about ready to ditch the whole ECU system and go with an aftermarket unit. Before I drop that sort of money, does anyone have any ideas?

Thanks for your help.

Also, what are the part numbers for the MAF Power and Burn-off relays. I have searched this board but have found conflicting results as to if these are interchangeable. Some people say they are totally interchangable, others say no they aren't. Any definitive answer? I believe my power relay is currently an MR22. However the listed part number now appears to be MR82. Could this be the sourse of my problem?

 

Check your fuel pressure. Sounds like a pump

 

As I mentioned above, the fuel pressure is fine. The pump has been replaced along with the filter and fuel pressure regulator diaphram. Neither of these replacements seemed to help the problem.

 

Under what conditions to you see the problem most often occur? Cold or hot? Does it all the time? Just some of the time? After driving a specific distance? Totally random?

Might be the ignition module going south.... just a guess.

 

It happens under all conditions. Temperature doesn't seem to affect it. That said, most of my recent driving has been in a temperature range of 30 degrees. It doesn't seem to take a certain distance. The only time recently when the problem has GONE away is when I have replaced either the ECU or the TPS. It definitely disappeared but within a day (or few hours of total driving) it returned. The coil, module, cap and rotor are only a few days old as I replaced them.

 

So you tried pulling codes?

Im not big into the electronics/EFI junk but sounds to me like it might be getting a crap reading from a sensor somewhere.

 

So what's the ACTUAL FUEL PRESSURE while it's ACTUALLY DOING IT? Not the fuel pressure while it's sitting idling; but WHILE DRIVING DOWN THE ROAD, experiencing the problem. I.e., what the fuel pressure REALLY IS, as compared to WHAT YOU WISH IT WAS.

Replacing parts doesn't guarantee that the fuel pressure is OK.

Your description sounds like a textbook case of fuel starvation.

I think the thing about fixing it temporarily with ignition parts is classic butt dyno recalibration. You want so bad for it to work, that you think it actually does, until you can't ignore the evidence to the contrary any longer.

Tape a fuel pressure gauge to your windshield. Go get on an empty stretch of road and make it screw up. Tell us what the fuel pressure is REALLY DOING while this is going on.

I'm betting you have 40 years worth of accumulated crusties clogging up the fuel lines inside your gas tank.

 

I will try and do that with the fuel pressure guage however it will certainly be hard to do with the hood design and whatnot.

After replacing the ECU the first time, it was definitely a different vehicle. There was no hiding the fact that the problem did go away. Same with the TPS replacement. While I dont doubt it could be a fuel starvation problem, there is no question in my mind that after both those replacements, the problem did temporarily go away.