1997 XLT surges at idle when aircon comes on

[maxzebra]

The car is stock standard except hi lift rear springs, and has done 289000 kms. It has recently begun surging from idle speed of 500 rpm upto 1200 rpm when the aircon clutch comes in. The dealer has had several goes at fixing this but can't. You are likely to ram a wall or pillar in a parking lot, or back end someone at the traffic lights. You never know when a surge is coming. They say the computer puts on more power for the aircon - and that's right, but it has been mapped correctly until now, and was recently reprogrammed with the latest FoMoCo updates. But it is still as bad as ever. Some sensor has gone kaput, or there must be an airleak I reckon. Any suggestions??????

 

[witchdoctor]

My 98 did virtually the same thing, and as it turned out, it was from the thermostat going bad. I got that from a thread somewhere on here (don't remember which). As soon as I changed the Tstat the idle returned to normal.

 

[Glacier991]

Sounds vaguely like either a bad IAC, or (oddly enough) a bad tps. I am assuming no codes are being thrown. I'd try unplugging the tps and see if that helps (THAT WILL throw a code, but you can have it reset). WHY Do that ? Well if the TPS is the culprit (a guess on my part, and knowing my guesses probably wrong) unplugging it will force the computer to use derived numbers for it... the car will run ok... but IF The TPS WAS bad, the derived numbers will eliminate the glitches in the TPS, and the surge should go away. Now mind you, that is IF the TPS was the problem. If it wasn't you'll be cursing me because NOW you will have a CEL on. You might reasonably ask why, if the TPS is bad (staying with my half baked theory) WHY wouldn't it throw a code? By all rights it should, but oddly, they often don't.

When you turn on the AC the only sensor/component that kicks in (gets activated) that affects idle speed is the Idle Air Compensator (IAC). That seems an obvious culprit, but I'd not expect a surge to 1200 RPM out of it. Still has it been removed and cleaned? Has the dealer investigated the IAC ?

Anyway those are my off the top of my head thoughts.... Hope they might help you. Let us know what you ultimately discover.

 

[maxzebra]

Many thanks for the information. What do you mean "tps"? I don't know that abbreviation. As a matter of fact, since Explorers were introduced only in 97 and are not very common in Australia - Ford have done an appalling job of marketing them - the dealers know "diddly squat" abut them. I had to go to six dealers to get a reasonable demonstration,and I bought it 350 kms away from where I live!!!!!

The result of this is that the dealer has not done anything to the IAC valve, and I suspect that this is the real problem after reading the various threads and posts. Another car I had with EFI had a bad "extra air valve" which is probably the same thing as the IAC valve here.

Now I can get the dealer to do the thing they should always have done. Or do it myself!!!!!

Manythanks!

Max Williams

 

[maxzebra]

Thanks Witchdoctor. If the IAC valve turns out not to be the problem, I will give the Tstat a go. But the coolant temp looks solid as a rock, as it always has been, and at teh original OK temp.

Cheers,
Max Williams

 

[maxzebra]

Just a note for all those who have helped me. The IAC valve turned out to be the problem. A good cleaning with throttle body cleaner did the trick. The return spring on the valve was all gummed up, and there was a heap of rubbish that came out as particles (some quite large) of grit. There is an air bleed hole on the side of the valve body, and it is protected by a filter made up of four layers of felt. Needed a good clean as well. The butterfly valve (a balnce valve???) in the manifold was also pretty gummed up and required a greater than normal torque to move it. I guess that was not working properly either. Again - all clean now, and the engine now operates properly, and runs much better. Thanks guys - Fantastic!!!!