91 240 wagon high rev / surge

e777 · Post by **e777** » 21 Feb 2014, 19:22

My wife's 240 wagon (240k miles) has been prone to random idle surges. The the car will be stopped, in gear, and with the brakes on, then the engine will rev pretty high (no tach so I can't say how many rpm's). She says that when this happens if her foot were taken off the brake, the car would surely lurch forward very quickly.

I have researched this in various forum archives, but have found nothing conclusive. The car is not producing any codes, but the check engine light has been on lately. Based on my research, I have checked and eliminated the following possible causes:
-throttle body and throttle position sensor
- broken intake hose
- MAF sensor
- intake gasket
- recently put in a new idle air control valve
- all vacuum hoses around the intake area, flame trap and connecting hose

Any suggestions?

BocaBryan · Post by **BocaBryan** » 24 Feb 2014, 05:53

Just a thought, have you checked the coolant temperature sensor?

Is there any strange behavior with the temperature gauge right when it happens?

I had this problem and it was a shorted out coolant temp sensor, causing the computer to think the engine was cold and sending more fuel. That was in my 1997, so I'm not sure the 91 works the same way.

e777 · Post by **e777** » 24 Feb 2014, 09:31

Thanks very much for the response. I will check it out!

e777 · Post by **e777** » 30 Apr 2014, 05:46

UPDATE: So, after months of having random high-rpm idle surges along with the check engine light constantly on, I finally solved the problem, thanks to the advice of forum member BocaBryan.

I replaced the coolant temp sensor (not the sender). The old one was rather pitted and oxidized. With that, the idle surges have disappeared and the check engine light went off. Mind you that all this time the car was not throwing codes of any kind.

Since the coolant temp sensor is difficult to access, I decided to just pull of the intake manifold to facilitate access. This gave me the opportunity to renew the intake gasket.

Thanks again, BocaBryan!

wagonlove940 · Post by **wagonlove940** » 03 Nov 2017, 17:05

I have the same happening in my 940, it will also idle so low on decelerating and stall completely. Engind temp sensor was changed so I will be looking/hoping the intake manifold gasket is what's causing my issue