Okay, I'm at my wit's end with this, so I'm coming to y'all for help...

Old truck, 200k miles.

Started after a fill-up a month ago, thought it was water in the gas, put in some heet - no difference. Ran several tanks through, with heet in each, to no avail. But, it's basically running just like it has bad gas.

It never seems to do it for the first couple miles, only after it gets a little warmed-up. You can hear it out the exhaust (high flow exhaust system), so I thought it was maybe running very rich, however, I've since convinced myself that I'm just hearing it not firing, rather than hearing combustion in the exhaust system.

I thought maybe I had fuel flow issues, although the fuel filter only has a couple thousand miles on it. My concern was maybe that some sediment was loose in the old tank, and clogging the pickup. So, I rigged up a fuel pressure gauge where I could see it, and hit the road. Whether the truck is missing or not, it has 30 psi at 15"+ vacuum (idle), 40 psi at 0" vacuum (full throttle), and the appropriate pressure at all conditions in between, from 1000-4000 rpm, so that pretty much tells me that the fuel pump, filter and regulator are okay.

Also, the pressure holds for several minutes after shutting down, so that would seem to rule out a stuck injector.

I checked the primary and secondary resistance on the coil, and I believe they were in spec with what the Chilton's book said, although I can't recall the specific numbers right now.

Tune-up parts are all good - brass contact cap & rotor, wires are fine, plugs properly gapped.

I took the ignition module to a parts store, and had them test it. Their tester had three lights - low RPM, high RPM and SPOUT. It passed the test three times. So then, we took the heatsink off and tried three more times. Same result, pass.

Whether it's in good-running or poor-running mode, it always starts immediately and idles well, with a rock-solid 17" of vacuum at idle.

Also, it does this at basically any part throttle condition more than about half throttle - 7" of vacuum. It also does it at WOT.

Fuel mileage is poor, but then, it always has been. This truck is always towing an enclosed trailer. The mileage seems like it may be slightly lower, but it's not outside of the range that it always has been. 7-9 mpg. I check it every tank.

I know I can test the voltage at the TPS, and I intend to, but it looked like it's hard to get at, and the only time I have to work on it is when it's dark. Also, I was thinking that maybe the MAP sensor could be an issue. It's hard to work on anything during farm season. Right now I work from before sunrise to after dark, and the only decent parts store in the butthole town I'm staying near is only open until 7 pm. I had to RUSH to get in there tonight to get that module tested. Sundown at 5, worked till 6, had to stop and pick up some paperwork on the way back into town, etc...

Please share any insight. Thanks in advance.

Any insight?

 

i had a similar problem on my 1987 300 six but it would only miss under par-throttle.
turned out to be the TPS

i have 2 different ways to test, both easy

1. disconnect the TPS
when i did this it ran better that it had literally in years no miss smother idle the whole 9 yards.
2.under the hood just barley push on the throttle.
if ur TPS is going bad it should start to miss a little and the RPM will fluctuate some. Like a "dead spot" in the ignition
I have seen this once on the 300 six and once on the 302 with a miss under par-throttle problem and both times the TPS was the culprit.

 

Thanks for the reply, I'll give that a shot when it gets light out, haha.