EFI trouble!!!

[Broncoman1969]

I know a lot of you have put the EFI 351 truck motor in your Broncos. I am having trouble with the '93 EFI motor. It has always seemed low on power. A tune up helped some but still does not have the power I think it should have. The other day when I was driving down the road it started having what seemed like a lean backfire through the throttle body. As I nursed it back to the house I found that it would run better once I got the rpms higher. That seems backwards to what I was thinking the problem was... low fuel pressure. I decided to park it and look at it in the morning when I wasn't so upset. Anyway, in the AM it started and ran fine. Still low on power but not backfiring at all. Any ideas? Checked codes and all it showed was throttle position sensor (which I had already replaced. Old code?) and egr valve not opening. Beyond that nothing. I don't want to take it out and get stranded so basically it is stranded at the house. Any ideas? Help please. Thanks!

Karl

 

[Broncoman1969]

I was wondering if the temperature might have something to do with it? BTW, When I first got it I did a tune up on it by replacing the dist cap, rotor, wires, and plugs. That helped some with the original low power problem but now this is a new problem. Could it be a temperature sensor that knows when it is hot and changes the fuel, ignition or ???? Could it be the computer causing the trouble? I am baffled by the EFI stuff but am good with the carbs and electronic ignition. Guess there is just a learning curve on this... I'm behind that curve I guess.

 

[AZMike]

Having the TPS out of adjustment would make it run like crap. Make sure you have it adjusted to .8 - .99 volts at idle:
1) Unplug the Idle Air Bypass (IAB) electrical connector
2) Set the base idle where you want it using the adjustment screw
3) Turn the motor off and loosen the 2 screws that hold the TPS on
4) Adjust it so the voltage reads .8 - .99 volts in the idle position.
5) Plug the IAB back in

If it still throws fits, you've got something else going on. Backfiring could be a lean condition. Some things to check for (in order of easiness):
- Negative pressure in the gas tank
- Clogged fuel filter
- Vacuum leaks
- MAF sensor dirty - use some good electrical cleaner on it
- Oxygen sensors going bad (you should get codes here)
- Vapor lock - if you have no low pressure pumps in or near the tanks
(I'm still battling this one)
- And of course, wiring issues!

 

[justinoshea]

Start with Mike's advice. If still broke, buy this book and you can solve any EFI problem;
Ford Fuel Injection & Electronic Engine Control
by Charles O. Probst

Read it five times, then find the strategy(s) that you are having problems with, and check those sensors & mechanical parts.

 

[Broncoman1969]

Thanks Mike. My TPS is one of the non adjustable types. The screw holes where it mounts does not have those adjustment slots. I am going to see if replacing it again will make any changes since that is the code it is still throwing. Also I think I will pick up a new fuel filter when I go buy the TPS and replace it again. It can't hurt. Out of curiousity, how would you cure the negative pressure in the tank? Shouldn't the system be a sealed, non-vented system in the original F250 anyway? The truck has always "sucked air" when I removed the gas cap. Not that it matters, as long as it runs good, but wouldn't having a loose fitting or vented cap also throw a code? Also, thanks Justin, I will look for and pick up this book!

 

[AZMike]

Thanks Mike. My TPS is one of the non adjustable types. The screw holes where it mounts does not have those adjustment slots. I am going to see if replacing it again will make any changes since that is the code it is still throwing. Also I think I will pick up a new fuel filter when I go buy the TPS and replace it again. It can't hurt. Out of curiousity, how would you cure the negative pressure in the tank? Shouldn't the system be a sealed, non-vented system in the original F250 anyway? The truck has always "sucked air" when I removed the gas cap. Not that it matters, as long as it runs good, but wouldn't having a loose fitting or vented cap also throw a code? Also, thanks Justin, I will look for and pick up this book!

Venting the tank properly would eliminate the negative pressure. It could be as simple as getting a different gas cap, or it could be more complicated than that. The bottom line is that you wanna eliminate the easy stuff first. If you put a fuel pressure gauge on the fuel rail and open the cap and the pressure shoots up to normal, that's your problem. If the pressure is normal with the cap on, this isn't the problem. Fuel pressure at the fuel rails is the key here. It should be in the upper 30s.

Most TPSs should have some adjustment built into them. Before spending another $30 for a new one, try to get yours set up correctly first. You should be able to open up the screw holes with a dremmel or drill bit to get the voltage you're looking for.

I'll second the advice on the Probst book - with one exception - read it more like 10 or 15 times!

 

[Broncoman1969]

I guess the difficulty here is adjusting it with the TPS on the engine. I guess I can remove the throttle body and adjust it then install the throttle body. You can't get at it with the throttle body on the intake. Any idea the cheapest source for the book? Amazon was a rip off. The cheapest price listed was $75 for a used one up to $175!!! Thanks Mike!

 

[justinoshea]

$24

http://www.themotorbookstore.com/ford-fuel-injection-1988-1993.html

 

[scottynelson2]

I am have tps problems as well. Can any figure this one out, voltage is at .01 and just stays there no matter where the thottle is. I replaced it with a junkyard one same thing, so I went out and bought a new one same thing. It runs and drives Ok for the most part, It just doesnt want to start when its warm and shuts its self down on long hills. When you unplug it the idle changes for a second so the computer is picking something up. Could I have a bad ecc or wiring problems and how would I check?

 

[COBlu77]

This should help with troubleshooting the TPS http://sbftech.com/index.php/topic,1031 . Although it's pretty tough to really identify a problem like this on a forum, after reading both of your posts, my bet is on fuel delivery/pressure issues. When my TPS went bad and went out of range, my computer went into limp mode and raised the idle to 1200 rpm, when the TPS was back in range it idled fine. The bad TPS did not affect anything but idle.

 

[scottynelson2]

cool Ill give it a shot thanks for the tip

 

[AZMike]

You can also check to make sure you're getting the right voltage to the TPS VREF wire. It should be 5 volts. If not, you have a wiring or computer problem. To see, check the resistance between the sensor and the appropriate pin at the computer harness. You might have a loose connection causing the problem. Here is a list of pinouts.

If you're running the stock 5.0 harness, there are 4 sensors that all share the same circuit - I believe they are TPS, BP, ECT, and ACT. Another one of those sensors failing could potentially case problems with the TPS reading as well. If there is a major problem, you'd see codes for several, if not all of these sensors.

It could be fuel or wiring as well. I'd get a fuel pressure gauge and check that out first as that's a lot easier to troubleshoot than wiring. I've had the joy of going thru my entire harness several times to get all the gremlins out of it. I can say that I now know a whole lot more about fuel injection than I did a couple of years ago!

 

[bigmuddy]

At risk of sounding like an idiot how do you check resistance between the TPS and the pin? I have voltmeter but I am not fluent with using the little thing/

Ben

 

[AZMike]

First, you'll probably need some extra wire. I would also highly recommend a digital VM if yours is analog. You put the VM into resistance mode (you're gonna be looking for less than 5 ohms) and hook it up to the circuit.

The easiest way I've found is to back-probe the wire at the sensor with a sewing needle and then attach your extra wire to it with an alligator clip and run the other end to near your computer. If you want to get really accurate, measure the resistance in that section of wire first so you can subtract it out later.

You should then be able to put your VM probe into the computer plug at the appropriate pin and touch the other end to your extra wire. No real need to remove the computer if you can back-probe the pin at the computer plug, but it just depends on how much access you have there. You want less than 5 ohms of resistance in the wire. Anything more and you've got some sort of connector problem somewhere in the circuit. Less than 2 ohms is a more typical reading, so I'd even be wary of 5 ohms.

 

[Broncoman1969]

Mine is back to running as it was before. I am suspecting a dirty connector but of course until it goes out completely I can't find it! Gotta love those computer controls!

 

[AZMike]

The "salt and pepper shaker" connectors are another spot that is notorious for causing electrical problems too. They should be located somewhere between the back of the motor and the firewall. You might wanna give those plugs a good cleaning and adjustment.