I post this in hopes that there is someone out there that may have something else for me to test before I pull my thin hair out.

The truck - 1990 F150 4x4 302 AOD - 120,*** miles

Here's the issue:
Since I first bought the truck it has had a kind of random miss at idle. Pulled plugs to replace and found #8 porcelain badly eroded. All plugs look like they were running very lean.

At the same time of discovering these issues I installed headers and an exhaust which I'm sure further leaned it out.
And in this hunt for the issue along with just wanting to replace stuff on this old truck I have done the following:
New Distributor
Coil
Wires
Plugs
ECT Sensor
ACT Sensor
MAP Sensor
TP Sensor
O2 Sensor
Fuel Filter
Air Filter
Fuel Injectors

The truck had recently had a "major" tuneup when I bought it and someone had replaced the ECM. Not being able to identify which unit was in it by the aftermarket part # on the case, I replaced it once more with the appropriate computer for this trucks engine/trans combo (just to rule out having the wrong calibration computer)

Now, no codes are present and no check engine light.
At light throttle it will kind of sputter. As if the accelerator pump weren't working on a carb.
It will also buck and miss under a heavy load at the same throttle position (as in light throttle in OD)
Also, as you accelerate under load it will ping like crazy and just doesn't feel quite right on power.

What I have tried:
Checked fuel pressure while driving
Driven with Spout out base timing 10 degrees (ping still present)
Looked high and low for vacuum leak
Cleaned EGR
Did the carbon bust thing with Seafoam
Installed Breakout box and watched TPS voltage while driving (nothing speradic or dead spots)
Check all kinds of wiring and sensor circuits with breakout box.
Replaced plugs once more and found #8 to be showing erosion again
Compression Test - All Cylinders 175-185lb's
Re-routed PCV to main vacuum Tee with new valve

I am getting desperate here. Does anyone have anything else that I could possibly look into?

Any input would be greatly appreciated,
Mike

 

What fuel pressure are you getting?
Are you sure that the EGR valve is closing completely?

 

I'm getting 32 psi at idle - up to right about 40 psi driving down the road under load.
Egr has been cleaned. Also tried a home made block off plate underneath it before which has been removed now.

For some reason I keep being pulled back to ignition. But, with new coil and complete new distributor (new TFI etc.) - what else could that leave in the ignition system?
Especially that would influence the problem to be in the #8 cylinder the most?

Mike

 

Perhaps a little anti-freeze is getting into #8 from a small head gasket leak?

Maybe do a leakdown test on the cylinder and pressure test on the cooling system.

 

I had a truck that did exacty as you describe and the air injection system was the problem. If the system is still intact I suggest you remove the tube that runs across the back of the motor for a close visual inspection, if it's in rough shape either replace it or eliminate the whole system and plug the holes in the heads.

 

I tried disconnecting the hose between the pump/valve and the tube that runs up to the T behind the engine. No change noted.
Is there something else back there that would have a different effect than disconnecting as I did?
Thanks,
Mike

 

If the system has any holes in it that's when the motor runs lean.. seems strange I know you would think fresh air either leaking into or being pumped into the exhaust would make it run rich but it doesn't. But in any case if nothing changed when you disconnected the air pump then that sorta confirms there is already a hole in it somewhere. It's near impossible to see behind the engine so that's why I suggested you just pull out the crossover tube.. there are only 2 bolts holding it to the back of the motor. On my truck it had rusted and there were pin holes in it, but I have seen a broken/cracked check valve cause the same problem.

Here's a pic of the tube from another post, you can clearly see the holes in this one.

 

ok, Thanks.
I will look deeper into this and post back results.

Will the air tube cause a mixture change only in the 4 & 8 cyl or will it effect all?

Mike

 

It will affect all cylinders because the air changes the O2 sensor readings and the computer treats all cylinders the same. The fouled plugs on #4 and 8 are because of the location of the PVC vacuum line at the back of the upper intake, I believe there was a service bulletin to move this to the vacuum tree and plug the port at the back of the intake.

 

Ok, unfortunately I do not think the Air injection system is my problem. This is a 1990 and this stuff looks like it could come off of a couple year old truck!



I gave it the blow test and the 1-way valve is working and the tubes are clear with no holes in them.

Here are some photos of my spark plug issue as follows:
Photo 1 is #8 Plug that I pulled out when I first bought the truck
Photo 2 is #8 I had put in and pulled with maybe 3K Miles and Headers added
Photo 3 is what all 7 of the other cylinders look like when I pulled photo 2 plug
Photo 4 is a plug from my wifes explorer (nice tan coloration) as I think these should look. I think it is running lean all around?

Tomorrow I will tinker some more
Mike

 

Yeah that tube looks like new, but those plugs are the worst I have ever seen. something is making this motor run excessively lean, are you sure you don't have a 5.8 computer on this motor?

 

When I bought the truck someone had replaced the ecm and I was not able to identify the model by the rebuild part # at the time, so I bought another one of the correct calibration: 302 4x4 AOD just to rule that out.
Same problem.

One thing I do have a question on is the O2 sensor. I replaced that when I installed my Y-pipe and you have the option of a 3 wire or a 4 wire. The truck had a 3 wire so that is what I got. Later looking at it there are 4 wires in the harness on the truck.
Is this common for Ford to have all 4 wires in the vehicle harness and use a 3 wire sensor?

Mike

 

A 4 wire set-up uses an internal heater in the O2 sensor. Don't know why a 3 wire plug would be the same as a 4 wire plug.

 

What's the calibration code on the one you have now?

 

I do not know the "Ford" Calibration code. It is a Standard Motor Products for F150 302 4x4 AOD Ca Emissions.

Ok, here is where I am now:
Really gone over the wiring to each sensor checking resistance between harness plug and break-out box. Everything seems just fine.
O2 wiring seem'd sketchy so I pulled the haness completely out from by the battery down to the sensor and inspected carefully. Interesting that Ford changes wire colors on that section of harness. I thought at first the maybe someone switched some wires around but they all check out location wise to the pin-out diagrams in my repair manual and factory wiring diagrams.

Still ? the 3-wire sensor on the harness with all 4 wires in it. The 4th wire would be a Sig-Rtn?
Would that have any effect on the readings the computer sees?

I swear up and down there are no vacuum leaks unless I am just not catching something but I think it would have to be pretty major to cause this much of an effect.

Last thing I tried today and don't laugh. Got a good deal on a 5.8 eec on ebay so I threw that in and it completely eliminated the flat spot off of idle, Greatly increased overall power, and greatly lessened the ping.
This is getting very frustrating. Obviously the engine need more fuel. But, If the 302 Calibration should be able to provide it, why is it not?
I've done a tripple take just to make sure someone didn't swap in a 351 in place of the 302. What else could possibly cause this?

Mike