map sensor questions/problem

[Crashtest84]

So my truck has started running bad. 89 with a 91 4.0 swapped. "been running for 4 years" so this is nothing to do with that. Symptoms started as a slight pinging on the hwy under light excel. Didn't ping at WOT. I just went with it until it got very bad pinging so badly that I was worried about burning a valve or piston. " this happened on the way to work". Later the truck died and didn't want to restart after checking for spark and fuel I found that the rail had no pressure after cranking. " possible bad fuel pump" so I jumped the pump test pump worked and has ever since. So now after using a mac scan tool. code 22 and 54 are what its giving me. But for the MAP sensor it reads 14 psi and never changed on the live readout. I can upload a video of that possibly. So i haven't bench tested the sensor. But a new sensor had no change. So i'm a little stumped and going to start checking the wiring along with testing the sensor on the truck. Anyone have any input on this or ran into something like it ?

 

[RonD]

'90-'92 4.0l did have BARO/MAP sensors but ran MAF sensor as the main air flow data collector, it wasn't the same as the MAP sensor on say the 2.9l using Speed Density system.
MAP was drop in '93 because it wasn't needed, MAF did the job for higher altitudes which was the concern.

Depending on where you live 14psi may be normal, 14.7psi at sea level, and unless you drive up into the mountains it won't change much.
1,500ft elevation is 13.9psi


Pinging is the engine running Lean, or some very low octane fuel.

If you had 14psi B/MAP and were driving at 5,000ft elev then engine would be running very rich, can't really cause too much of a lean out at 14psi.

Same with IAT(intake air temp) it has a very narrow range so can't see it causing pinging, it can only adjust the fuel mix slightly, and it would have to be showing very HOT air temp to go any where near lean

What does surprise me is the lack of Lean code???
Pinging means lean so O2 sensor should have pointed that out to computer, and then computer would set a code when it tried to correct for it.

That part doesn't make sense, O2 sensors last 100k miles, after that they can cost you money in fuel, because they usually fail lean, causing engine to run rich.

'91 should have two O2 sensors, one on each bank, so both failing would be long odds.

I would have a look at the spark plugs, see what they tell you.
Also if possible do a compression test, all spark plugs removed for the test.
4.0l is 9.0:1 ratio so should come in at 160psi, if it is higher, 170+ you could have some carbon build up which increase compression and causes regular gas to pre-detonate.

I would also see if you can get an old timing light, and check the spark timing on #1, '92 does use a separate EDIS-6 module for spark control, could be a problem in that module.

Most pre-'95 non-Calif 4.0l didn't have EGR systems, EGR system helps prevent higher temps in cylinders, which causes high NOx emissions, but side effect of having an EGR system is less pinging from an engine so spark timing can be more aggressive.
If you have a EGR system check it out, if not then never mind

 

[adsm08]

1) The baro PID didn't change because on that year the reading is taken when the key is first turned to run, then the sensor is ignored until the next key cycle. It is a massive failing of the software design, and it is what causes problems when making drastic changes in altitude. Pike's Peak racers using old EFI systems often run into this, the engine will either start running very rich, or lean out and stall depending on which way you are going.

2) Pinging can indicate a lean condition. It can indicate other things as well. It can be a timing issue, not likely on a 4.0, but not impossible if a cam or crank sensor, or even the EDIS module is failing. It can indicate cross-fire on the plug wires or, and less common but still theoretically possible, inside the coil itself. It can also be caused by carbon build-up in the cylinder. A deposit on a valve face, or on the piston, can cause a hot spot that will ignite the mix too soon. Many early EFI engines had an "octane rod" that could be removed or installed to indicate to the computer if the engine was running on 87 or 89 octane, and an issue in that circuit could cause ping. I don't know for sure it was present on your donor vehicle, but the possibility exists.

3) I have yet to see in person, or see any indication in service manuals/wiring diagrams, a pre-93 4.0L with EGR. From what I can find 93 was the first year the Cali car 4.0s had it, and IME all of the EGR equipped federal 4.0s before 95 were in Explorers. The rule that seems to apply to the Rangers of those years was no SEFI, no EGR. That's just going off the wiring diagrams I used to wire up my swaps, there may well be some odd-ball units out there that don't conform to that.