I have a soft spot for OBS Fords, it seems I never learn: I bought the truck below from a tow yard for a 'good deal' and... well... we all know how that goes sometimes: Gamble and win or gamble and lose...
Anyway, I'm enjoying the ride and learning a bunch, so I'll consider it a win either way.

1987 F250 5.0 NP435 2wd - Diagnostic

• Fuel pressure - 32psi idle, 39 w/ vac line removed
  • Pressure rapidly lost after pump primes or engine shuts down, 0 psi retained
  • Running the rear tank fills the front tank
• Vacuum - Lots, stable 17+in hg
• Timing - 10-11* w/spout out, computer appears to advance timing properly
• New Accel distro cap/rotor, wires, FoMoCo Platium plugs (folks seem to have a neg opinion on these?)
• EGR - Blocked @ intake & manifold, electrics still attached
• New CLT/IAT Sensors
• New TPS (old one tested bad)
• IAC removed, cleaned, re-gasketed. Seems ok, felt like a 'cheap' part however.
• MAP does not throw codes and swapping out w/new did not improve operation (so I returned it to save 60 bucks.) I do think I should check the vRef, but testing in-range during code check encouraged me to look elsewhere.
• Right now I've pulled PCM and will open it to look for leaky caps when I can source a nut-driver/socket tiny enough. The case screws are smaller than 4mm.
• Stock Cats in place, chamber style 3" single cat back

Codes
KOEO - 33
KOER - 33, 44, 52
No surprise on the 33 (EGR), or on the 52 since it smells pig rich, the 44(thermactor fault) I take to be a non issue at this time. Correct me if I'm wrong. (EDIT: 52 is Power Steering Switch open. Was having a mental moment.)

Symptomatic
Truck Idle- will purr like a kitten at 750-850ish RPM, or hang at 2500, or chill at the 1100 range. Sometimes when it's hung up I can 'kick' the throttle and idle will settle back like an old carb rig coming off high-idle. I've looked for vac leaks, but when the idle quality is SO good sometimes, I'm inclined to think it's something else.
Truck Chugs - does ok off idle, makes very low power whenever 'asked' to do anything like accelerate or go up a slight incline. It will not cruise evenly at part-throttle under, say, 2000RPM. I haven't managed many miles under these conditions, but I can confirm fuel economy = poor.

Proposed course of action
Regardless of the outcome of ECU inspection, I clearly have a fuel system problem. Does it look like a failed fuel tank selector valve? I think that can cause the fuel pressure to be lost at shutdown. Stuck injector? Both? I think I know what I want to do, but before I spend the whole weekend reeking of gasoline, I'd like to weigh in with the experts.

 

Well I'm certainly not an expert, however as far as I know when your fuel system won't hold pressure it is usually a stuck injector. A stuck injector will also cause a rich condition. Fuel pressure of 32 and 39 with vacuum line disconnected sounds about right. I think it should be 40 to 42 with the vacuum line disconnected but I'm not 100% sure. One tank filling the other is a fuel selector issue. The suspect fuel delivery module is the one that is filling the other tank so your rear tank fuel delivery module needs to be replaced.

The driveablilty issue may be a clogged cat. I don't know about the idle issues but a clogged cat will give the exact symptoms you describe. Truck will usually run fine at idle or low speeds but when "asked" to do anything the truck will bog down, have a serious lack of power and get very crappy mileage. I tested mine by checking the pressure at the exhaust outlet and having someone else raise the rpm to about 2000 or so. In my case I couldn't feel any appreciable difference in the volume of exhaust gasses coming out of the pipe. Gutting the cat solved all of my drivability issues.

 

The tank valve sounds problematic, with putting fuel back to the wrong tank. Wasn't there a fix to install check valves to keep that from happening? Or was that newer trucks, 92-up when they changed that system? Can't recall the details, just remember parts of it.

The IAC still sounds like maybe an issue with the idle all over the place. Since you replaced the TPS already. Seems sometimes the cheap replacements aren't as good as a cleaned OEM model? I used to have to play with my '89 truck constantly. Swapping OEM and Azone units around to keep it happy.

That's all I can add.

 

Thanks fellas - I thought about the clogged Cat deal and think it has some merit. I think I'll plop the PCM back in there and drive it to the exhaust shop and have 'em 'delete' it (sold my welder, dammit) until I know what direction I'm taking with this rig. I'm hoping to bypass the fuel selector valve and run off the front tank only until I get a chance to do a 38 gal rear tank swap and only have a one tank system.

 

Yeah my truck is pre "fuel delivery module" - as of now, it has a low pressure pump in each tank and a high pressure booster along the frame. I'd say with 100% certainty the fuel selector valve ($55 or so) is a problem, I just am not sure its THE problem, I don't really want to do injectors, but I may just hafta.

 

This model truck has the older style fuel system. There is no "fuel delivery module" causing the wrong tank to fill from pressure relief. This has the "dual tank fuel reservoir" problem. So, it has three fuel pumps and the pressure-driven switching valve that causes the issue. Either rebuild the existing valve/reservoir or get a new one.
Can the frame mounted high pressure pump cause the rapid pressure loss on shutdown? I don't know. Haven't heard of that, but why not? More likely bad injector or fuel pressure regulator - even new ones can be bad off the shelf nowadays.

 

I only know my FPR is ok at idle, not sure under driving conditions. I *think* it is probably ok. I do not have a hand vac pump to further test it, but I can get one.

I agree that my truck has the older style fuel system w/3 pumps. I will re-read the diagnostic steps in my Chilton book, but I think a failed fuel selector valve can cause both:

1. Loss of pressure retention (not as sure about this)
2. Fuel return being routed to the wrong tank.

 

Aha, I forgot about the change with the fuel delivery.

 

May be worth noting - truck starts great: hot, cold, or otherwise. (many leaky injector posts reference hard starts)

 

Closing my own loop on this -

1. Installed newer 4-hole injectors from JY. Old ones looked pretty gross anyway.
2. Did a new FPR 'while I was in there' since it's easy with the fuel rail off and was $19 peace of mind
3. Double checked the routing of my spark plug wires and... dammit. I hate to admit it, but I had cyl 3 and 4 mixed up as the leads crossed over in a way I wasn't seeing on the passenger side. I suspected cross-fire or mis-route all along, as the truck just sounded like it wasn't hitting on all cylinders under load, but well... no matter how many times I double checked...I guess it was amateur hour. Trust your gut, fellas! (Also, any future replacement plug-wires will be Motorcraft numbered leads...)
4. With IAC connected, truck still hangs an idle anywhere from 1000 to 3000 RPM which is pretty humiliating. With it disconnected, truck runs perfectly, idling under 1000 and pulling smoothly throughout rev range. I'll likely replace the IAC w/a quality part.
5. I did open the PCM and... it passes a visual inspection! No leaky caps or broken leads. I guess it's OK for now.

To wrap it up, I'm glad to have "swapped" my 6 cyl back to an 8 cyl The JY injectors were worth my time either way. I've learned boatloads.

On to the next thing...

 

please if you ever figure out the erratic high idle issue ,let me know --have been wrestling with this for months -- Ive got a 87 302 also --cannot pull codes even with an obd1 tester - replaced about all sensors including IAC ??

 

At present, I am just enjoying driving a truck that runs on all 8cyl - and just in time since I have a household move coming up - aka not a bunch of time to spend wrenching until I get resettled. Since my idle is rock-solid w/o the IAC, I'm going to leave it dis-con for the time being and replace it with a high quality part at a later date.

• Why can't you pull codes? Is your harness or computer damaged?
• Vac leaks seem to be the most detrimental threat to idle quality.
• Can you check mechanical factors: Fuel Pressure, Timing, Idle Vac?

 

Hi , I had lots of issues with my 86 302 sd , I did get it pretty good , I put a wideband oxygen sensor in the collector, .......one thing you mentioned was the fuel pressure , if you have an adjustable regulator i:e bbk I think loosing pressure is normal . I found fuel pressure make a huge difference with mpg. I had high idle issues as well as truck idling slower and slower and stopping this was the fuel mixture getting very rich 9 or10 to 1 . The high idle problem I found the replacement pcv passing too much air and make alot of noise , swapped for a universal one .So less air from that means you can alter you base idle a bit and give the idle air less to do and this seemed to stop mine going up and down . I think there is a sweet spot for those two . Make sure the regulator is hooked up to the correct vacuum source . . Cheers.

 

Hey thanks NZ - I've got er running pretty good at the moment, but definitely still have something to chase since it runs far better with the IAC disconnected. I'll keep the rich condition in mind, but my FPR is a factory non-adj versus a bbk or something similar. Appreciate your ideas!

 

I also had a lot of sort of misfiring , where it just didn't sound right not on 8 , fitted a new ACCEL coil then it was great . My engine is in a intermediate stage ( as i am building a lower compression 385ci w ) and has pretty high compression from 1966 d ported small chamber heads 10.88 to 1 on a flat top 87 roller block,0 decked . Cheers