I have had this 940 for nearly 4 years. In the last 2 weeks it has started to idle erratically. This happens when the car has been running for a while and the engine is warmed up. When I put pressure on the accelerator when taking off at the lights or going up a steep hill the engine idle surges up and down a few times till I get to the top of the hill or if I'm at the lights gently feather the accelerator till it settles down.

When I first drive the car around it runs fine.

My local mechanic has replaced the air flow meter, the ECU and has assured me that he has checked other possibilities but the problem still occurs. Any ideas would be greatly appreciated.

Regards CPG.

Replace your IAT sensor. It's a very cheap part and could be your problem. Even if it's not the problem, it could fail down the road.

I went thru many parts on my 1994 940 start/idle problem on my own before my Volvo mechanic found it was the IAT.

Look closely at this picture:

http://www.brickboard.com/FAQ/700-900/Exhaust.htm#AirInjectionValve

I had poor performance as well as failed the GA EPA smog tests. Took it to a reputable Euro specialist and they said the only thing they could do was slightly alter the camshaft timing with the camshaft/timing belt.

I started snooping around under the hood and could hear a slight pulsing air hiss, which turned out to be the three outlets in the rubber hose in the picture.

On my '94 940, the front fitting (closest to the radiator) on the rubber hose had a slight split and was leaking. The only part source that I could find was the dealer so bit the bullet and replaced the "hose". Problem solved. I also purchased 3 hose clamps and placed one around each of the three openings.

Problem solved and no more problems in 4 years.

I think the '94/95 model years were the only engines with the "pulse air" feature. Owned a 1975 Cosworth Vega that employed the same pulse air technique, but metal pipe/fittings were used in lieu of the rubber hose.

I would also clean the throttle bore and throttle blade. Another thing to check would be the thermostat inside the air filter box, these tend to fail allowing the inlet air temp to go too high, at its worst this can lead to air flow meter failure, at best the air density is too low for the air flow meter to compensate and causes rolling/low idle speed.

Dear CPG,

Hope you're well. The first thing to do, when idle speed varies randomly, is to check the On-Board Diagnostic codes. See the site's FAQs for how to do this and post back with any codes.

The next thing to do, is to inspect and - if necessary - clean the idle air control valve (IACV). If the IACV is dirty - black, sooty deposits inside the tubes, to which the hoses connect - it needs to be cleaned.

The IACV is mounted on a bracket, beside the intake manifold. It has two rubber hoses at the end closest to the front of the engine bay. These hoses are secured with screw-operated clamps; if factory-original a 7mm socket is required. There is an electrical connector at the end closest to the firewall. Press on the release bail and pull gently to remove the connector.

Keep the valve's nose pointed down, so that any solvent used does not get into the motor compartment. If you use cotton-tipped swabs, be sure all fibers are removed. If you want to immerse the valve in solvent, put it in hose nose down, and limit solvent depth so that solvent does not get into the motor compartment.

ECUs are the last thing to replace: they rarely fail. If the air mass meter had failed, it would have set a code.

Your mechanic may be a random parts replacer, rather than an analyst. Such mechanics are a cause of much wasted money.

Hope this helps.

Yours faithfully,

Spook

Thanks for taking the time to respond Spook. I'll carry out the work you described this weekend and if I find any codes I'll let you know. It will be interesting to see what state the IACV is in.

Regards CPG