Hey everyone,

I recently bought a 1988 f150 with the 300 (EFI). I am having a couple issues that i'm hoping someone could help me with!

At first start up, the truck idles real rough and the RPM's jump back and forth between 500 and 1500. Sometimes it drops so low is stalls itself out. When the rpm's jump back and forth, I can feel the brake pedal slightly move in out (assuming the vacuum?). After about a minute or two, the idle (though still rough) slowly levels out at around 750-900 RPM.

If I try to drive the truck before it levels out, I have to give it lots of gas while disengaging the clutch or else it stalls. Once I get moving, without giving it proper "warm up time," the truck chugs, jerks, bounces, surges, and misfires. Again, after a few minutes the truck runs fine with no issues.

However, sometimes when I get to a red light, the truck will idle level (though rough) at 800 then randomly fall to 500, then surge itself up to 1000. Any ideas what all these issues could be?

I have replaced the plugs, wires, distributor cap, air filter, and EGR valve. I also replaced the fuel pressure regulator which fixed the problem for about a week. Since the problem has come back, I have cleaned the IAC valve and throttle body, and seafoamed the truck to no avail.

When I unplug the IAC valve sensor, the truck drops in idle and stalls. When I disconnect the MAP sensor, the truck starts then stalls, so I am assuming those are ok.

The truck sat some time before I bought it and only has 60k original miles.

Any ideas! Thanks!!!

 

Unplug the electrical connection on the MAP sensor, or the vacuum hose? If you unplugged the vacuum hose, stalling/running pig rich is the correct behavior. If you unplugged the electrical connection, it should start up and run.

I'd start by first pulling the codes.

 

Hey,

When I unplug the electrical connector, it runs... maybe even smoother than with it connected. When I disconnect the vacuum, it starts bouncing idle and stalling.

I removed the throttle and cleaned it out real good. Also replaced the TPS. Truck still has the same problem, and actually has a higher idle now. AND now the check engine light is on.

I'll have to get a shop to run the codes tomorrow. Until then, any more ideas?

 

Check for codes yourself: Fuel Injection Technical Library » How To Run a Self-Test

 

Do the fuel pumps run continiously when you turn the ignition switch to "on" (not all the way to start the truck) ????

Bob

 

The TPS should of fixed it in my opinion. Did you get a bad Sensor? Are the wires screwed up that go to it?

Check for Vacuum leaks. Spray some carb cleaner around suspected area's. Start taking off and plugging vacuum lines one at a time just to get an idea while it's running.

I'm no expert but to me that still sounds like a bad TPS.

 

Yeah I was almost 100% sure after process of elimination that it was the TPS. Just to update, the problem on start up is not necessarily when the truck is cold. I drove the truck around for about 15 minutes (hoping the check engine light would go out), turned it off then started it back up 10 minutes later. Same thing happened: fluctuating idle and stumbling in first and second gear. After about 3 minutes it went away.

This leads me to believe a fuel pressure problem? I replaced the fuel pressure regulator and that fixed the problem for about a week. Maybe faulty FPR?

 

I never had an issue with a FPR on our model trucks so not sure.

 

Check the codes, the truck is trying to tell you something if the light is on.

 

did you set the tps to the right voltage ?
check the codes.

 

There is no "setting the TPS to the right voltage". If the output voltage at closed throttle is out of range then you need to determine if the throttle blades are open too far/too little or more than likely, you have a faulty TPS. If the output is out of range or way under the value for open throttle once again verify the throttle blades are opening completely. If they are, you have a bad TPS.

cut from oldfuelinjection.com:

The TPS sensor has a great deal of influence on the fuel ratios and timing curves, because the TPS is a user interface sensor. The TPS is the closest thing you have to a phone call into EEC. EEC listens to your every command from the TPS. There is a TPS ratchet algorithm which continuously seeks the minimum throttle angle corresponding to a CLOSED THROTTLE position. This alleviates the necessity to set the throttle position sensor at an absolute position and compensates for system changes and differences between vehicles.

•Closed Throttle can be anywhere between 0.6-1.0 volts
•Part Throttle is triggered @ 0.04 volts above Closed Throttle
•Full Throttle is triggered @ 2.71 volts above Closed Throttle


Here is a link to the entire article Ryan has on the TPS and it's operation:


Fuel Injection Technical Library » Throttle Position Sensor (TPS)

Setting the TPS to some magic closed throttle value is an urban myth that has been debunked many times. If you have to elongate the mounting holes to bring the closed throttle output within range there is problem that lies elsewhere. Fix the root cause, not the symptom.

 

So i ran the codes today. Got a KOEO and CM code of 63: TPS below .6 volts. That would explain the check engine light and the 1450 warmed up idle? How do I fix this one? Did I install it wrong?

Also, I have a theory to my rough idle/running at startup problem. The truck runs rough during "open loop," or it's predetermined fuel/air mixtures before it warms up and receives information from its sensors. Maybe something is wrong with the vacuum or fuel pressure initially and it smooths out when the sensors adjust for it.

So my question would be...Do sensors adjust for a vacuum leak or low fuel pressure?

Also, what if the Engine Coolant Sensor is telling the truck that its already warmed up, thus immediately going into closed loop, causing a rough idle and missing until the truck has actually warmed up?

 

MAP/Speed Density trucks like this generally do not care about vacuum leaks as far as actual engine running quality goes. Vacuum leaks will cause a high idle though.

You'll want to get a multimeter and verify the TPS output voltage.

As far as coolant temperature goes, an ECT stuck "hot" will generally make cold starts very, very difficult. You can try unplugging it and see if the default fuel table makes it run any better.

 

Thanks for the link RLA. Great information! So it would appear I have an "under travel" rotational angle. Which way would I change the mounting of the TPS to increase the voltage? When i removed the old TPS, I had to drill out the mounting screws because they were rusted in. Im guessing my new drilled mounting holes are out of alignment?

 

Yep.
But all of this applies only IF the the EEC is functioning correctly.....hence the question I asked, "Do the fuel pumps run continiously when you turn the ignition switch to "on" (not all the way to start the truck) ????

If you get to the point where you've done all the "checking" you can without finding the problem, check your EEC.
Click this link:A9x ECM's (and same years ECM's) Failures Due to Age

Understand that the capacitors used in your EEC will fail with age. That's the nature of the electrolytic capacitors that were used in our EEC's.

I checked my EEC and it had the leaking capacitor problem, which I repaired.
The leaking electrolite can and will damage circuits on the circuit board (ie. unexplained poor performing engine operation as well as computer controlled transmission problems).
A leaking capacitor is a failed/failing capacitor.
If it's failing it will send poor information to or from the CPU.
If it is a failed capacitor, the circuit will not perform correctly and will send erronous information to or from the CPU.
You should consider all of this in your "repair stragety".

Think of it this way, if you looked under the hood and saw a corroded shorting wire, you'd take steps to correct, repair, or replace it, then check for other problems.

As time goes by, more and more EEC's will start to fail due to age.

It's worth a check, if not just for the peace of mind.

That's my take.

Bob