4.0 OHV Persistent Hard Start. Looking for ideas

[Kirby N.]

MAP sensor or barometric pressure sensor. I haven't checked that at all. I can get a new off brand one for $60 or a junk yard unit. Which would you recommend? I am going to swap today.

 

[Kirby N.]

Thanks for the idea. I’ll report back.

 

[Brain75]

Do you have a haynes or chilton manual? I don't want to "read the manual" to you, but I skimmed the troubleshooting, pondered, re-read your first post, pondered and I am back to fuel or air - before it actually starts it is basically brain dead, the computer plays little role before it catches and starts.

Hard to start HOT has very few causes which is why I kinda latched onto fuel system - It's basically all fuel when that problem shows up...

The first post says there is an issue with it loping. That is the one thing that doesn't point to fuel or spark plugs (and you said you have thrown all new at both of those systems basically).

I'm about to say you got multiple issues.
From what you have done and where you are now, I would check the whole air intake system quickly then check the fuel injectors - swap em out with another set if you have some. The reman on those is very good you almost never get junk reman'd injectors I understand (still running on factory here)....
anyhow here's the troubleshooting page from Haynes:

 

[Kirby N.]

Thanks- I’ll read that. I do have multiple Haynes and Chiltons for various rbvs but there are a lot of differences through the years. I have had a ton of rbvs so I saved the manuals. I also have a fsm for a 93 that includes just the chassis and the engine book- but it is a little different also- no map sensor anywhere in there.

I have done a lot of reading in there and pondering, and a lot of fiddling as a result, but haven’t come up with a good solution.

 

[Kirby N.]

I have a compression gauge. That is on the list to check compression. Perhaps today I can do that.

 

[Kirby N.]

Do you have a haynes or chilton manual? I don't want to "read the manual" to you, but I skimmed the troubleshooting, pondered, re-read your first post, pondered and I am back to fuel or air - before it actually starts it is basically brain dead, the computer plays little role before it catches and starts.

Hard to start HOT has very few causes which is why I kinda latched onto fuel system - It's basically all fuel when that problem shows up...

The first post says there is an issue with it loping. That is the one thing that doesn't point to fuel or spark plugs (and you said you have thrown all new at both of those systems basically).

I'm about to say you got multiple issues.
From what you have done and where you are now, I would check the whole air intake system quickly then check the fuel injectors - swap em out with another set if you have some. The reman on those is very good you almost never get junk reman'd injectors I understand (still running on factory here)....
anyhow here's the troubleshooting page from Haynes:

View attachment 120146

Also when I did the engine I put in remanned injectors that were flow matched and 5 hole. I did not keep my originals as I believe I sent them in as a core. I could probably get a junk yard set and swap them though as a test. That might be the next step.

when you speak of the computer being dead at start up, that makes a little sense. Initially when it tries to start it sounds like it almost will, then it doesn’t and goes into a hard start crank forever. I could just be thinking into it, but perhaps the map isn’t inputting altitude on that first crank to get it to start and that is part of it. Idk. I’ll try it today. Thinking junk yard unit first.

Thanks again

 

[Kirby N.]

And I could totally have 2 issues. I have been fiddling with 2 different tps and 2 different iac units and the idle seems to have some differences with the 2 different sensors.

 

[bobbywalter]

dont buy a sensor. test the signal and make sure it is reporting in range. swapping the sensor with one from a known working vehicle if you can, is easier for sure...but a bad wire or corroded connector could be the problem as well.

where it is located can be water intensive ect.

i dont have data close for that setup, but the charts are out here.

the injectors for sure are suspect.

is your truck bank fire or sefi? does it have a cam sensor?

i assume it is bank fire.

how fast does the rail pressure bust off? if it is instant then the regulator or check in the pump is not working well.

this can contribute to slow start and aired up rail can cause stumble for a minute especially if there is a flow restriction in the rail.

restriction issues will be more pronounced under power so that seems unlikely.

theinjectors being partially clogged or faulty or sticky when cold explains alot.

the baro reporting incorrectly could be pulling enough fuel, it may be too lean to start correctly requiring some throttle....or cam sensor causing a long crank.

pretty sure you dont have a cam sensor...but cant know without seeing the thing

 

[Kirby N.]

I don’t know if it is bank fire or sefi. I actually don’t know what that means. It is waste spark from what I understand. 3 coils and it uses each for two cylinders and sparks extra when it sparks for the other cylinder.

I already bought a map sensor. I got one from a junk yard this morning. It was $5, so I figured worth the try. The dealer doesn’t have a new one and didn’t find anything online for a motorcraft unit. I can get an aftermarket one srill

I’ll have to find my notes for the fuel pressure test. I don’t remember it decreasing quickly.

I haven’t done anything with the valve on top of the gas tank. It does have a new ford pressure regulator and I did check it with the fuel pressure test and there is no fuel in the vacuum line.

 

[Kirby N.]

Fuel pressure numbers-

Key on engine off (cycle a few times because I bled off pressure getting it hooked up)
- 38psi

Key on engine running - 31psi

After sitting for 10 minutes- just below 38.

After sitting for an hour- 30 psi
2 hours- 23psi

When I key on again to start it, it jumps back to 38 psi. While cranking then to 32 when it starts. With the fpr vacuum unplugged it goes to 39, plug back in to 31.

 

[Brain75]

...
the baro reporting incorrectly could be pulling enough fuel, it may be too lean to start correctly requiring some throttle....or cam sensor causing a long crank.
...

Ya learn something every day - I did not know this... and that explains a ton of what is likely going on here.

 

[Shran]

Should hold fuel pressure indefinitely. Curious what would happen if you let it sit overnight with the gauge hooked up? The pump in my 94 Explorer is really weak and will bleed off to zero pretty fast, often I have to cycle the key 4-5 times before it will cold start and then it usually runs out of fuel & dies once. Once it's running, it's fine.

Bad FPR usually causes hard hot starts and your numbers seem OK so I guess I would rule that out. I'd expect to see more like 35psi at idle and up to 40 briefly if you blip the throttle but you're not way out of that area so it shouldn't matter much.

I was leaning towards MAP/barometric pressure issues myself so let us know on that. 1991 should not have a cam sensor (probably why your 93 ECM threw a code, it may have expected that or EGR or both.)

Have you swapped out the coil pack with a different one? Long shot, probably not the problem but just one more thing to try.

 

[Kirby N.]

I swapped the map sensor for the used one and reset the computer and it doesn’t make a difference. Still starting hard.

It does not have a cam sync sensor. It does not have egr. That is why it is a little puzzling. I am kind of out of running out of options.

Thanks for chiming in @Shran. Cycling the key many times before cranking makes no difference. It hard starts cold or hot. Doesn’t matter. The only time it starts ok is if it has been running and I turn it off and crank it immediately with in 1 minute. Anything longer and it starts hard.

 

[Shran]

Just for shits you could try giving it a nice big shot of ether and see if it lights off right away or still struggles. A hard start there would (in my mind) indicate something with timing/spark/compression/valve train. Instant start would lead me more towards fuel, injectors, a ground on something... do you have a good strong reference voltage reading, IE at like the TPS? Should be 5v and shared with just about every sensor in the system.

 

[Uncle Gump]

I'm a stickler for Motorcraft plugs... ate the replacement plugs Motorcraft?

Have you put a spark tester on it?

Those coils should be capable of about 60kv. The spark should easily jump that gap with an audible "snap"

Also... the more you crank... the more fuel you add. Try going to clear flood mode if it doesn't fire up on the initial crank. My thought is you could be flooding the engine.

What did the plugs look like when you changed them?

When you do get it started you say it idles poorly. How dies it run up top?

I've heard a lot of complaints on replacement injectors. To bad you didn't retain your old ones. I would certainly be looking for some factory injectors from the bone yard.