I bought it from my brother after he had a new short block installed at Galpin Ford. (not impressed with the quality of their work) I have been able to rectify a number of shortcomings in this rebuild, but cannot for the life of me track down why the idle fluctuates (when in N or P) between a low of about 600 RPM to a high of 1100 RPM. Note that the short-block used was a Motorcraft. Also, I had them install Smog system friendly after market JBA headers rather than factory exhaust manifolds��.hey, they were less expensive!

Believe I have corrected all of the vacuum leaks, installed new cap, rotor, plugs and wires (wires were old and really bad,) new 02 sensor which has helped the rough idle to some degree. Also, canister was crushed, air pump ��silencer�� was broken at the neck and the EGR Valve has been replaced. Local dealer (Drew Ford, outstanding service dept. BTW) cleaned out the injectors.

Now I still have the fluctuation in idle speed as well as what I consider to be a rough idle for a late model FI vehicle. Shouldn��t it run pretty smoothly rather than like my Tiger with a ��race cam? For my money, I always thought you could tune a good 5.0 or 5.8 to run smoothly enough to safely put a glass of water on the thing and have it sit there with few ripples. Maybe I am expecting too much. Gas consumption is pretty high as well. No apparent problems with power on the road or idle fluctuating when stopped at a light. Though the idle is a bit rough at a consistent 750 to 800 rpm. When taken to dealer and friends shop, no apparent indications of a sensor failure that is noted by their equipment. Is it time to yank out the ECM and have it looked at?? Truck had 98K when the rebuild was performed, now has a bit over 99K. Another friend (who restores British cars and builds Ford racing motors,) suggested that the Catalytic converter may be clogged, but I doubt that as it passed smog and has what seems to be plenty of power.

Sorry for the long post. Appreciate sincerely all thoughts and suggestions.

Thanks for your reply and I will perform the check.

Beleive you are talking about the sensor that is at the base of the throttle body and has three wires going into it. Two questions:

1) Do you check the voltage with the key on only or with the engine running....I assume the later

2) Any idea of which wires to probe. Assume no ground as the ground should be the throttle body. Unfortunately, my manuals do not give this kind of detail.

2. I looked, but I can't find the color ids for the wires, sorry.

Hopefully someone else has it?

Steve,

Yes, the throttle body seems to be clean. I am not sure what you mean by the IAC motor, but I assume you mean the Idle Air Switch (not sure about this) but located on the throttle body in between the two air intakes? If so, I checked this out the other day....removed the solenoid or motor and sprayed carb cleaner into it (with it open by pressing on plunger,) until it ran clear. No change or help to speak of. Still surges and idles rough.

In a previous post the suggestion was made to just replace the ECM as they are cheap. Checked into this and depending on the model for the 5.8, they are anywhere from 33.99 to 49.99. May remove it, but before doing so will check the grounds at the connector for any shorts or open circuits.

Thanks once again for the advice!

1) Checked themactor, found rear unit missing the cap and spring. Replaced it.

2) Checked NEW spark plug wires. #3 and #4 insulators were melting off...did have arcing to headers on #4. Replaced with JBA wires and put heat shields on #3 and #4. Idle now substantially smoother (Duh!)

3) Checked voltage at TPS and found it to read at 1.3+ volts. Replaced it and got down to 1.024 volts. Idle is not adjustable as this is a Califorina vehicle. Bent tab as far as I could to get down this low without binding. Bending stop on old TPS did nothing. Cleaned back of throttle plates in the process.

4) Replaced Idle Air Bypass valve and solenoid....had to put the old solenoid back on because I have not had a chance to reverse the leads (as noted in Tech Ref.) because of in-line diode and new unit has built in diode.

5) Have NOT replaced ECM. Unfortunately around here, they are still quite spendy at nearly $200.00 each. Pulled it and some minor corrosion on the case. Most places told me that you will know it is bad cause the engine will not run at all....don't know if that's true or not.

6) Removed factory exhaust from the CAT back and replaced with 3 inch system and 3 chamber Flow Master to improve breathing. Also sounds nice, but not too loud. Save my Tiger for LOUD. :-)

All around, it does idle much more smoothly now, but still surges occassionally between 600 and 1100 RPM, rather than idling smoothly at 750-850 RPM.

Don't know what else to check to calm this thing down. Guess there is a possibility it could need a new fuel filter (as I think it is original,) or possibly the MAP sensor.

Any further suggestions or tests I can perform? Thanks for any help you can offer.

Regards,

Allan

However Ford ECMS VERY RARELY go bad. I have replaced almost a dozen GM ECMS and quite a few Asian ECMS but never a Ford ECM. When I work on a Ford now I don't even consider the ECM as a possibility. If we call our tech line about a Ford one of thier catch phrases is that the ECM is very unlikely to be the cause, so keep that in mind.

Of course Alex has more experience with these systems than I do and I don't mean to contradict what he is saying.

I'd take a look at the O2 sensor; if it is still original then I would recommend replacement. Vacuum leaks are hard on Oxygen sensors and many times when the leaks are fixed the O2 has been out of range for so long that it doesn't read correctly anymore. This could easily account for your poor fuel mileage.

Hope this helps some. Good luck

Steve and Charlie...thanks for your replies.

Charlie, replacing the O2 sensor was one of my first steps and it did not seem to affect anything. I think I will however re-check all of the vacumm lines again.

Steve, I have not performed that check yet....really wish I would have thought of it. If the engine dies, that means no vacumm leaks...if it stays running and stumbles, that means I do have a pretty good leak somewhere if I recall....I have this right, don't I??

Someone also said the MAP sensor might be at fault too. Thoughts?? Fuel Filter ??? (very important in and EFI system,) and finally, what about the EEC sensors over on the right hand fender well (Near the VIP testing terminals) ??? Any thoughts that these might have been affected due to the dead short at #4 plug?

Thanks for helping me out with this puzzle.

Our pleasure, we're here to help.

Steve, I have not performed that check yet....really wish I would have thought of it. If the engine dies, that means no vacumm leaks...if it stays running and stumbles, that means I do have a pretty good leak somewhere if I recall....I have this right, don't I??

Exactly. You should be able to stall it right out.

Someone also said the MAP sensor might be at fault too. Thoughts?? Fuel Filter ??? (very important in and EFI system,) and finally, what about the EEC sensors over on the right hand fender well (Near the VIP testing terminals) ??? Any thoughts that these might have been affected due to the dead short at #4 plug?

The fuel filter would show itself as a surging at a steady cruise speed, as well as limiting power. Am I correct in assuming that aside from a hunting idle, the truck performs well?

I blocked off the Throttle body last night and it died immediately....so I am pretty sure I have no vacumm leaks to speak of.

Plenty of power when at speed and when I put my foot in it, so still not sure about the fuel filter.

Interestingly enough, while toying with the MAP sensor, I noticed a couple of interesting things. 1) when vacumm was removed, the idle really dropped, but with no surge. 2) When I removed the electical connector to the MAP (with vacumm line back in place,) the engine went up to about 1100 RPM and stayed there with no surge at all. Could it be I have a bad MAP sensor or sender for that matter?? Don't have anything that will measure frequency, so I may just have to take a risk and replace the MAP.

Once again, thanks for your ideas. I WILL get this sorted out one day soon.

Regards,

Regards and I will keep you posted,

quote:

---

Originally posted by Allan:
Additionally, he said the the the 5.8L is pretty notorious for having vacumm leaks where the plenum joins the intake manifold. However, I have sprayed carb cleaner profusely around the plenum and with no change noted. A new MAP sensor it is.

---

And you can stall the motor with your hand, so you have no vacuum leak.

Good luck, I bet the MAP sensor does it.

Sorry; I meant to check this thread sooner but I forgot.

It sounds like the MAP sensor could very well be the problem. When you unplug it the computer uses a default value and if the idle smoothes out you have a real cause to doubt the sensor.

All Ford map sensors that have the same plug can be used to test your's; so you might find a friend with a Speed-Density Mustang or another Truck, 302/351/300 doesn't matter. Explorers, Rangers, you name it. It might be a better idea than throwing down $65 for a new one before you are sure.

If you have a multimeter you can check the MAP sensor, however I don't have any specs for you. I will try to get you some by tommorrow night if you haven't already fixed the truck. It will range from ~ 5.00 - 0 volts I beleive, with high numbers at idle (high vacuum) and lower voltage when the throttle is opened quickly.

Good luck

Thanks for your reply and it is indeed time for an update.

Yes, these plemun gaskets are notorious for leaking on this model. It indeed may be the problem.

In frustration, I finally took the beast to a trusted Ford dealer. They said they couldn't find anyting wrong with it other than it appears that the Throttle is sticking ever so slightly open and sugested replacement with a new throttle body to the tune of $394 for the body alone....no labor.

Did some checking around and found someone who rebuild throttle bodies to the tune of $270. Will get it to him tomorrow and back at the end of next week.

The reason I took it to the dealer is that I took the beast on a long trip last Saturday and it only got 6 MPG at constant freeway speeds!

The guy that rebuilds these things said that both the surging idle and the mileage will be corrected with the rebuild. We'll see. The next place to look is for a plemun gasket leak and then to "flow" all of the injectors.

I'll get it eventually. Thanks to all for your help.

Regards,

I hope you find the problem!!

Took it the throttle body to the fellow who was to rebuild it. He tinkered with it a bit and noticed that the plate on top of the body where the cable attaches was binding with the body. Said someone must've dropped it. Bent it back, said it was perfect and "no charge."

I got a new gasket, calibrated the TPS to 0.9 volts and the surging idle is gone for the most part. Though it STILL does not idle as smoothly as I would like, the idle is none the less steady. He said that having that plate hang-up occasionally would cause intermitent surge and poor fuel economy as well.

I won't know about the mileage until I fill the tanks, but it seems to be better. The proof will be in the fill-up.

Just a side note. The Ford dealer, which has a very good service department (and the mechanic that worked on it is the son of a friend,) stated that these trucks from about 1987 to 1991 never really idled very smoothly at all, no matter WHAT they did. Apparently the old-timers stated people would bring them in all the time when they were new, complaining of idle roughness and there was nothing they could do about it.

Oh well, we'll see. I am going to drive it for a while and see what happens.

Thanks all for your advice....keep it coming.