HELP! Can't get people to fix my 1988 F150


I have a 1988 Ford F150/2wd/300six/5speed. The engine runs like its running too rich with all the symptons: Blackened plugs, rough idle, black smoke, etc. I’ve taken this vehicle to 5 different places and I’ve had a friend look at it. Each time I take it someplace they fix some stuff and charge me. I’ve spent over $1500 bucks and it still runs like a junker. All the normal stuff has been checked. I don’t know what to do! Some places say it has low compression. My friend tested it and says it does not (cyl press 175 - 187). Can someone help me? I really like this truck but I need to make it run.


Before anyone makes a suggestion it would be nice to know exactly what has been done to htis point.


Exactly what has been checked?

I’m unfamiliar with that specific vehicle, but I suspect it’s a throttle body injected system. The usual items controlling the fuel metering in a TBI system in '88 would be the O2 sensor, MAF sensor, MAP sensor, fuel pressure (read: regulator), temp sensor, and injector itself. And, of course, the ECU. And I probably missed one or two items. Any of these could cause you to run rich.

Additionally, a compression test, while a good thing to check, can be misleading. You have compression rings and oil rings. The compression rings could be still holding but the oil rings weak (lost spring tension), worn, or gummed up. In short, the cylinders can hold pressure from the top side but still let oil past the bottom side.

Oil can also leak past valve guide seals.

If you’ll post what’s been checked to date we’ll see if we can help. There are just too many places to start without more info.

Oh, if it’s carburated don’t be afraid to say so…I’m very experienced at retracting my foot from my mouth.

  • mountainbike


The first thing I would check would be the fuel pressure regulator. A leaking regulator diaphragm can cause all of those symptoms and they’re easy to check.

There are 2 more possibilities and they are related to the ignition.
Your truck uses the TFI-IV system and if someone has ever adjusted the ignition timing without disconnecting the SPOUT connector, then the timing will be way off. Seriously retarded timing can cause low power, bad mileage, rich running, etc.

The second would be if the ignition module is failing. The symptoms on these modules can vary but generally running rough, coughing, flat quitting, and running rich are some of them. The modules are a known problem also.

Also check the coil wire terminals. These can corrode and cause a weak spark. Maybe this has already been done?

I would look at that pressure regulator first and your compression readings are fine.


The first thing I would do is replace the Coolant Temperature Sensor (CTS). They are prone to fail and the resulting symptoms are exactly as you describe. CTS is cheap and very easy to replace.


You’re right. My bad.

Nearly everything has been checked, sensor wise, however, I have not heard any one of “my” mechanics mention the CTS. MAF, MAP and O2 sensors were checked and the fuel pressure was checked. All showed OK. There was an EGR valve that was replaced. Honestly, the CTS sounds like a good next step. If it was carbed, I’d say the choke was sticking shut. I can see where if a temp sensor thought the engine was still cold it would cause this. The engine runs betterr when you first start it but it gets worse and worse and it warms up. No evidence of oil leakage around either intake guides or rings shows up. Just too much gas. I’ll go after the CTS first and the fuel pressure regulator second.

To be honest, I’m the friend with the compression tester, not the actual owner and I’m hopping mad that these shops have screwed my friend so shamelessly. My friend wouldn’t be upset with the $1500 bill if they had just fixed the darn thing. But, no, they just would get stumped, say he needed to spend $2500 to $3300 for a new engine based on low compression and then charge hime $200 (or more) for the “work” they did. The more I think about it, the madder I get!

Two shops actually claimed the #4 cylinder to be “shot” and it would require a new engine. However, #4 showed about 180psi on the compression test, just about in the middle of the range of 15psi. One mechanic, who appeared to be honest but just not too thorough said is was a wiring problem. He’s the one who tested most of the sensors.


OK. We replaced the CTS. Still runs the same. BAD! What would you all suggest next?


Since fuel pressure tested good the fuel pressure regulator (& injectors) should be OK & not leaking.

But since checking for a leaking fuel pressure regulator is so simple I’d do it anyway.

Just run it for a couple of seconds, shut it off & pull the vacuum hose off of the FPR. If the diaphragm is leaking, you’ll find fuel in the hose & that would make it run very rich.


If all else fails, you could convert it back to carburetted form, using salvage yard parts. I’m assuming you live in an area free from emissions testing…


Are the MAF and CTS sensors showing the proper voltages for the the airflow and normal operating temps?

Has the ECU been checked for any trouble codes and if there are some, what are they?

You might see what a vacuum gauge shows for readings. Maybe the exhaust system is partially plugged.


Same as before; check the fuel pressure regulator followed by replacement of the TFI-IV module.


If the fuel injection is throttle body only, the injector is probably worn out and used to cost about a hundred dollars. Tomco was the replacement I bought for the 87 Tempo and the black spot on the snow stopped happening. The gas mileage went way up but I didn’t get enough fuel to start the car when the temperature went below fifteen degrees above zero. So I got a head heater installed, used it once and then moved to the Ca. coast. After having my transmission rebuilt in Pa. Should have had it done in Ct.


The spark plugs became fouled. Certain parts were changed. The engine still ran poorly. Did somebody forget something? Those spark plugs, once fouled, are STILL fouled. Cleaning the spark plugs won’t necessarily unfoul them, though they may LOOK good.


Check manifold vacuum and exhaust pressure. That truck uses the speed-density logic (MAP) as I recall and a restricted exhaust will lower intake vacuum, which in turn will indicate a heavy load to the ECU which will in turn dump much gas to carry the load. I have an old Ford diagnosis manual somewhere. Drop me a note with your www address and I will forward the pages on your truck.