90/91 4.0 OHV issues in my Choptop

[lil_Blue_Ford]

So, I still have some wiring to sort out that may or may not be influencing anything from my swap since this originally had a 2.9.

I’ve been trying to sort out why it’s not running it’s best and why it’s running rich. I’ve managed to get it better than it was but I recently pulled codes and here we are.

I’ll start with what’s been replaced…

ECU - this was mainly done because I’ve been running an auto computer in a manual but also my capacitors popped and burned a couple resistors on the board. Replacement is a reman for a 91 Ranger 4.0 manual.

O2 sensor - I couldn’t remember when it was last changed. Got a Bosch from the local AutoZone.

TPS - apparently I somehow used a 2.9 TPS and it somehow worked despite turning the wrong way? Either way, it has an aftermarket 4.0 TPS now.

IAC - threw it on just for giggles.

BMAP - yeah, apparently the early 4.0 wiring harness will plug into a MAP sensor but it’s not supposed to. Deleted the MAP sensor in exchange for a barometric sensor. This did give a boost to fuel economy and was my most recent fix.

Now… I’m back to getting a high idle. It also has a hot start issue if I don’t let the E-fan run for a couple minutes when I shut it down, when I try to restart it has extended cranking and sometimes takes a couple tries to fire. Have not yet done a smoke test for vacuum leaks but it’s on the list. Fuel economy isn’t quite what I think it should be either. Pulled codes and I’m not sure what to make of it. Tried to pop the vacuum line off the FPR but it was stuck on there good and I haven’t got back to checking because it’s been on and off rain and cold the past two days.

I got a code 67 on the O side which the book says is a clutch/neutral safety or AC wiring issue which I suspect is in the wiring issues I’m trying to sort out with the connections made between the 89 Bronco II harness and the 90/91 Ranger harness. Having a little trouble trying to sort that out despite having EVTMs for 89, 90, and 91 but I’m still working on it.

The other code was a code 14 on the C side which the book says is an ignition PIP fault. Initial google search tried to tell me it’s a bad TFI or distributor pickup, but this has neither, just the ignition module on the core support. One other thing I found suggested a problem with the crank sensor. Not entirely sure what to make of it.

Hoping someone has some suggestions of what direction to head or some input on the wiring. I’ll keep studying the manuals, for whatever reason I’m finding wire colors that don’t match the books and connections in the manual that don’t specify what connector they go through. Of course, none of the plugs I need to work on have a pinout page so I’m stuck searching through the book.

 

[Shran]

From what I recall some early 4.0's had both a MAP sensor and a MAF sensor but all 4.0's had the latter. I'm not really following what you did there. A lot of trucks from this era are confusing as far as the MAP sensor goes - some need engine vacuum and some do not and are just open to the atmosphere. I'm not sure what to suggest there - it would depend on what the ECM is calibrated for.

High idle and hard hot starts could indicate to me that you are dealing with vacuum leaks or a leaking fuel pressure regulator or both but your code 14 might be in play there as well. A little bit of Googling I did suggested that it might be caused by a misfire or could be an ignition module or crank sensor problem on a 4.0. I think you will need an oscilloscope to test either of those. Check your wiring and swap with spares if you have them.

I don't think I would worry about the code 67. IIRC you can get that depending on AC on/off condition and whether or not the clutch pedal gets pushed in during the test.

 

[gaz]

@lil_Blue_Ford
At what engine RPM does it idle once warmed up? Is your thermostat opening to maintain temp once warmed up? Any issues while driving; example bucking, surging, low power or a miss?

1, if no vacuum.leaks, then I suspect that your ECT sensor (the 2 wire unit) may be faulty; when faulty, the engine will stay in warm-up mode at a higher idle.
2, there is an idle trim proceedure but it is worth going through all the vacuum lines 1st as a leak is definitely gonig to affect idle.
3, what temperature thermostat are you running, have you verified that it works correct.

 

[lil_Blue_Ford]

From what I recall some early 4.0's had both a MAP sensor and a MAF sensor but all 4.0's had the latter. I'm not really following what you did there. A lot of trucks from this era are confusing as far as the MAP sensor goes - some need engine vacuum and some do not and are just open to the atmosphere. I'm not sure what to suggest there - it would depend on what the ECM is calibrated for.

High idle and hard hot starts could indicate to me that you are dealing with vacuum leaks or a leaking fuel pressure regulator or both but your code 14 might be in play there as well. A little bit of Googling I did suggested that it might be caused by a misfire or could be an ignition module or crank sensor problem on a 4.0. I think you will need an oscilloscope to test either of those. Check your wiring and swap with spares if you have them.

I don't think I would worry about the code 67. IIRC you can get that depending on AC on/off condition and whether or not the clutch pedal gets pushed in during the test.

According to the parts store, the 90/91 4.0 used a BMAP that is open to the atmosphere and not a MAP with the vacuum line. I tried to confirm with my EVTMs but they conveniently don’t specify. That all went away for 92. Apparently when I swapped the 4.0 in many years ago I didn’t do a bunch of things correctly so I’m trying to work through and fix things correctly now that I’m driving it more.

That was kind of my thoughts with the hot start/high idle issue, but the weird thing is that was never a problem until I started fixing things. I don’t have an oscilloscope and unfortunately don’t have spares. At the moment I’m short on money so it will have to wait to try and get new parts. I did repair a bad wire in the harness but I have no idea what that wire does, I just saw it was damaged and fixed it.

 

[lil_Blue_Ford]

@lil_Blue_Ford
At what engine RPM does it idle once warmed up? Is your thermostat opening to maintain temp once warmed up? Any issues while driving; example bucking, surging, low power or a miss?

1, if no vacuum.leaks, then I suspect that your ECT sensor (the 2 wire unit) may be faulty; when faulty, the engine will stay in warm-up mode at a higher idle.
2, there is an idle trim proceedure but it is worth going through all the vacuum lines 1st as a leak is definitely gonig to affect idle.
3, what temperature thermostat are you running, have you verified that it works correct.

When I first replaced the computer (and still had all the other random wrong parts on), it would idle around 900. Now it idles around 1,200 like it did with the auto computer and initial start all the way up to around 2k, then it sits around 1,500 until fully warm.

No real noticeable issues driving, it runs better than ever with enough power to squawk the 35’s.

Forgot to mention the fuel pump, tank, filter, and lines were all replaced last year I think it was.

Not exactly sure on the thermostat or what temp it runs at, my dash “gauge” doesn’t work. IR temp gun says it’s staying cool but my e-fan never shuts off once it kicks on. Planning on sticking a mechanical temp gauge in one of the heater hoses at some point.

Thermostat was new when I dropped this motor in like 2 years ago if I mind right, should be a 180* like I ran in my 2.9