Surging Idle

[Jtrum01]

HELP!! I am at my whits-end and out of money!

I have a 1988 Bronco II that I recently had a number of things repaired at my local shop; New wires, distributor cap, flushed fuel system and injectors, new intake manifold gasket, new passenger side exhaust manifold, new exhaust system and oxygen sensor, new Idle speed valve, new throttle positioning sensor.

My problem is this: The engine will not maintain idle after warming up and I have extreme lose of power under normal acceleration and the idle surging occurs between 500-1100 RPM's. This problem tends to be intermittent and If I let the truck sit at idle when problem is detected it will eventually correct and return to a normal steady 900-1000 RPM's. Also, I have check and not found any vacuum leak anywhere.

I did have the exhaust replaced about 6mo ago and they put a new oxygen sensor in then. When these problems started after I had all that service work done and the garage couldn't figure out why it wasn't running properly, i put another oxygen sensor in and the truck ran perfect for about 24hr.....Then back to the same idle problem.

If anyone on this plant can figure this out, I would greatly appreciate it!

I have completely restored this truck back to its factory original condition and have over $16,000 invested into it!

Thank you,

 

[Krafty]

I would have someone hook up a diagnostic tool to your computer and check for codes. there can be a number of things that cause a pour idle, one that comes to mind is throttle position sensor and the Idle Air Controller.

 

[BroncoJoe19]

The most common causes of a surging idle is a vacuum leak or sticking IAC

I agree... pull diagnostic codes, all you need is a paperclip and the ability to count to ten.

How to scan FORD on board Diagnostic Trouble Codes (DTCs) in your driveway

http://broncozone.com/forums/index.php?s=&showtopic=14269&view=findpost&p=74587

Howto locate a vacuum leak

http://www.fordforumsonline.com/forum/engine-drivetrain/94-howto-how-locate-vacuum-leak.html#post423

HowTo check your IAC

http://www.fordforumsonline.com/forum/engine-drivetrain/261-howto-check-clean-your-idle-air-control-valve-iac-iab.html

 

[Jtrum01]

Thank you BroncoJoe and Krafty! I did replace the Throttle positioning sensor and Idle Air Controller too. I checked and can not find a vacuum leak, but I'll check again. I haven't tried the Diagnostic Trouble Codes procedure myself however, when it was in the shop they did check the codes and said the oxygen sensor was flag as malfunctioning so, I did replace that too. Once I replaced the O2 sensor the truck ran flawless for 24hr then went right back to the surging idle problem and lack of power under acceleration.

 

[Krafty]

I would have a good look at the plug on your O2 sensor, clean up both ends of the connection really well and see what that does for you, normally the o2 sensor wont tell the computer its bad, it usually tells the computer what to do,

what I meant by the diagnotstic tool was a snap-on scanner, it will tell you in real time what your engine is saying to your computer while your driving, it will tell you what temperature your map sensor says it is, what your engine temp is, your throttle position and air-fuel mixture.

it is good for situations like this where you don't know where to start.

most decent shops have them, and even some high-schools, if you can get a hold of the automotive shop teacher in your area and see if he has one and if he can work it into one of his classes, usually for free.

 

[Rons beast]

I have found this problem on a few other vehicles, and my Beast. After dilligently checking all other sources, IAC, O2 senc. etc. I found an old EGR valve will be the problem. After carbon build up over the miles it will not always close completly, and cause a surgng idle, possible stalling, and set a code. On an OBDII vehicle the code is often 171 or 173 or both and has you checking for the vacuum leaks, or changing sensors.

If the EGR has alot of miles change it. If it isn't closing properly it will make the engine run lean, and the computer tries to compensate.

Sounds like you checked everything else.

Good Luck.

 

[Jtrum01]

I would have a good look at the plug on your O2 sensor, clean up both ends of the connection really well and see what that does for you, normally the o2 sensor wont tell the computer its bad, it usually tells the computer what to do,

what I meant by the diagnotstic tool was a snap-on scanner, it will tell you in real time what your engine is saying to your computer while your driving, it will tell you what temperature your map sensor says it is, what your engine temp is, your throttle position and air-fuel mixture.

it is good for situations like this where you don't know where to start.

most decent shops have them, and even some high-schools, if you can get a hold of the automotive shop teacher in your area and see if he has one and if he can work it into one of his classes, usually for free.

Krafty,

The reason I replaced the intake manifold gasket was due to a major oil leak on the backside of the motor. This caused and excessive amount of oil to drip down and coat that O2 sensor connector so, I did clean it thoroughly. I will try the diagnostic tool approach. This is truly a head scratchier and quite frustrating. The vehicle is practically brand new after the amount of parts I've put into it.......lol

Once again Krafty, Thank you for advice!

 

[Jtrum01]

I have found this problem on a few other vehicles, and my Beast. After dilligently checking all other sources, IAC, O2 senc. etc. I found an old EGR valve will be the problem. After carbon build up over the miles it will not always close completly, and cause a surgng idle, possible stalling, and set a code. On an OBDII vehicle the code is often 171 or 173 or both and has you checking for the vacuum leaks, or changing sensors.

If the EGR has alot of miles change it. If it isn't closing properly it will make the engine run lean, and the computer tries to compensate.

Sounds like you checked everything else.

Good Luck.

Thank you for the advice however, my truck is a 1988 2.9L and has no EGR system on it......:-(

 

[Jtrum01]

I've come to the serious conclusion! After being an Automotive Engineer for GM for 27 years and countless restorations under my belt this Bronco II is simple HAUNTED! I am now looking for an automotive expert who specializes in EXORCISMS!.....This is the root cause solution I'm left with

 

[Krafty]

how did you make out on the checking codes, and/or using a scan tool?

 

[Rons beast]

I reread your original post and have concluded that the problem occurs when the engine is in the "closed loop" mode. It has warmed up and the computer has gone to controling engine functions based on input from the various sensors, rether than from preset perimeters.

The logical conclusion is that the problem is from a false or incorrect input to the computer. The idle is ok when cold, so the ability to control idle is there. The computer is receiving info requesting an increase in idle then decrease. However the info is not asking for something outside the comps. perimeters, so it doesn't set a code....at least doesn't set the light.

There still may be a code or codes and they have to be found to proced in a logical order.

If the code points to an O2 sensor again, you may have gotten a bad sensor, ( known to happen), or there may be something damaging the sensors. You said the intake gasket was replaced. It could be that the sealant used on gaskets, and some solvents and cleaners, could damage O2 sensors.

If the code points to an O2 and it is replaced and the problem is solved only to resurface, then there must be something damaging the O2s

Bottom line ...have the codes checked as Krafty suggested. Post the results here, and we all will do our best to exorcize that demon.

May the Bronco Gods be with you.