Strange engine issue

AgInternational · Oct 31, 2017

So, my 91 supercab with a 4.0 has this wierd problem with the engine. Sometimes it will run just fine, then suddenly start to die and instantly rev up the engine again. Then it will slowly go back to idle. It is a bit disturbing when it happens while driving so it has been parked for over a month now. Also while idling parked outside it will sometimes just die then crank but no start.

This might be unrelated but I pulled some plugs to see if I was getting spark(I am when cranking, always) and noticed 4 and 5 were getting extra fuel. I can smell it coming out the tailpipe too.

Now I am just wondering how I can fix this situation since I am driving my 2000 super duty right now and diesel went up, gas went down. Any help appreciated.

RonD · Nov 1, 2017

Welcome to TRS

You need to clean the MAF sensor, needs to be done every few years, very easy to do

You also need to clean the IAC(idle air control) Valve, a little harder but also needs to be done every few years as engine gets older and burns a little oil.
How-to here, with pictures: http://www.explorerforum.com/forums/index.php?threads/how-to-iac-valve-cleaning-thread-w-pics.84220/

If IAC Valve has a coating of oil inside then change your PCV valve and make sure its hose is clean, also Vent hose from oil filler tube to Air tube

As to your specific problem, running rich, you will want to check the Fuel Pressure Regulator(FPR) vacuum hose
FPR will be behind alternator on lower intake
It has Fuel Return Line attached AND a vacuum hose, you need to check if that hose has gasoline inside it
FPR can leak fuel and it WILL be sucked into the engine via that vacuum hose when vacuum is high, like at idle

Other possibility if all the rich cylinders are on one side is the O2 sensor
O2 sensors last 100k miles or so, if engine burns a little oil then they don't last as long
O2 sensors use a Chemical Reaction to detect Oxygen in the exhaust, so they wear out like a battery does.
When they start to run out of chemicals they cause engine to run richer than it should so MPG goes down, it is worth changing them as they get older just to get best MPG over next 100k miles

AgInternational · Nov 1, 2017

I cleaned the maf sensor earlier this year. It has a new IAC from last winter. The fpr and O2 sensor have less than 10k miles on them. Only plugs 5 and 6 looked like a rich problem, the rest were fine.

When it has the crank but no start issue, I don't hear relays click or the fuel pump run when I turn the key to on. When it does start, I hear the relays and fuel pump make their noises beforehand.

I think it might be an electrical issue but I haven't found much yet.

Sent from my Nexus 7 using Tapatalk

RonD · Nov 2, 2017

The EEC(PCM) Relay in the engine fuse box is activated by the key in RUN or START position.
EEC Relay powers up the EEC(computer), Fuel Pump Relay, Spark, and Fuel injectors

The relays are standard 12v automotive relays, and can fail, and not expensive to replace, but more likely the ignition switch under the steering column is the issue.
Ignition switch is a slide switch, when you turn the key a rod in the steering column slides the switch into its 4 positions, ACC, OFF, RUN, START
Slide can get loose and so can the key cylinder that moves it.

Pull out the EEC relay and have a look at its contacts and the slots for corrosion.
Key off
Use a test light in the slots to find 12volt power, that's full time power from battery

Key on
Test for another slot that now has 12v, that will be from ignition switch.
Wiggle and turn key off and on to see if that slot works(has 12v) every time.

AgInternational · Nov 2, 2017

The eec relay energizes but does not power up the computer. Switching relays didn't solve the problem. The fuel pump doesn't come on to prime nor does the check engine light when the key is turned to the on position.

Sent from my Nexus 7 using Tapatalk

RonD · Nov 3, 2017

Then EEC Fuse or fusible link is bad
In engine fuse box there should be a 30amp fuse, #8 maybe
It provides the power the EEC relay sends out to those devices
It uses a Yellow wire from fuse to EEC relay base

If you have a volt meter test EEC base, yellow wire for battery voltage, it should have voltage 24/7, all the time

Make sure fuse and its contacts are good and that the yellow wire is not frayed

1991 may use Fusible Links instead of fuses for some power wires
Fusible links are short wires with smaller diameter that "yellow" wire in this case.
They will separate without burning, so act the same as a fuse does but not as quick to blow as a fuse, so like a "slow blow" fuse.
Fusible links would be connected to Battery post on the Starter Relay(starter solenoid) on the inner fender, there will be a few, alternator used them, maybe fuel pump power
Early '90's was when transition was happening from fusible links to fuses

AgInternational · Nov 3, 2017

The eec relay energizes

When the key is turned to "on", the EEC relay clicks. That means the coil side of the relay is getting power. The Fuse was not blown last I checked. The switched side of the relay is getting power but everything after it is not. I tried swapping the relays to verify the bad relay but other relays had the same result.

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I marked as follows:
Red-constant voltage
Blue- Switched voltage
Green-constant ground

RonD · Nov 3, 2017

The Reds don't connect, there at the fuse box

From fuse #8/30amp to the EEC relay is a separate wire, yes it should have 24/7 voltage
In 1991 it should be a Yellow wire from #8 Fuse to EEC Relay
And that wire also goes to Pin 1 on EEC(computer), thats the KAM power(Keep Alive Memory) power, like in the Radio, Radios also have KAM power for keeping Clock and Presets "alive"
This Yellow wire doesn't power the EEC just lets it keep some memory for next startup.
Thats why "they say" to disconnect battery to "reset" the computer, you are cutting KAM power, so computer(and radio, lol) have to re-learn settings

Fuse #10 60amp goes to ignition switch, then from there thru your Blue line to EEC relays coil, key on power
The Blue and green are just for activating the EEC Relay, they don't "pass power" on to anything
The other two wires on the relay, yellow and red, are connected when relay closes and they pass power on.

If you don't have power on the yellow wire at the EEC relay then fuse or yellow wire is bad

Power Out of the EEC relay is a Red wire, it is spliced several times feeding power to many things, when EEC relay is closed.
So if EEC Relay is working, key on and it closes but no power on the Red wire then no EEC, no Fuel pump Relay, no spark, no IAC, no injectors, no MAF

AgInternational · Nov 4, 2017

First off, I know how a relay works and I know how to read a wiring diagram. I posted on here because I was running out of tests to perform and was getting a bit desperate. I thought I might find someone who has had the problem before or something.

Now, I was performing an extensive wiggle test over the entire right side of the engine compartment and made my way to the battery. As I pulled on the wires, the small ground wire that comes off the negative terminal of the battery came out of it's poorly spliced(from a previous owner)connection and poured a bunch of corroded powder stuff out. So I cut it back looking for clean wire until it was too short to use. I was just going to cut the end of the big cable and solder on a lug terminal but there was corrosion in that wire too. A lot of it. I cut it back bit by bit for about 8 inches then decided it wasn't worth the work and bought a new one from Autozone. Runs great now, only turns of when you turn the key off. Grounds are important too.

Strange engine issue

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