"Hunting" in Volvo 740 GLT
 

I have a 1990 Volvo 740 GLT (auto). It has done 30k miles and, as
expected, is in very good condition. However, it has a problem
rendering it unuseable. It starts and idles normally but when the
throttle is depressed it starts "hunting". Take your foot off the
throttle and it again idles normally. Does anyone know what is likely to cause this?

The most likely cause is a split in the hose between the AFM/AMM and the throttle body or worst case scenario, the AFM/AMM itself. Also if the AFM/AMM is unplugged it will do the same.

Also check all your breather hoses for splits, security and also for blockages. Usually blocked breathers cause various symptoms including rough running so i would look more for splits, disconnected hoses etc.

Also check the idle control valve and the hoses attached to it. ;) :D

On a different note -
 

So she is a twin-cam then? On a car that is SO low miles check the age of the cambelt. It may have been overlooked because the mileage limit has not been reached. And take the balance shaft belt off anyway - and make sure that you have a steel oil pump pulley and a nice new correct Volvo bolt in it! Oh, and check the cambelt tensioner too. If anything happens to the cambelt it is bye-bye engine. And check the jubilee clips on the water hoses and preferably replace them - if a hose comes off on the motorway it is also very bad news. That's what my daughter did to mine - failed to see the change on the temperature gauge and THEN tried to get to the next motorway service station. Irretrievably cooked engine!

Many thanks Laird and Roger, will check both out and let you know how we get on. There are no split hoses so will try a new sensor.

"Hunting" in a volvo 740GLT
 

Just an update to the hunting problem. We have fitted a new MAF sensor but it has made no difference. Any other thoughts?

Inlet manifold gasket? Loose inlet manifold? FUBAR Lambda sensor? Split hose between inlet manifold and auxiliary air valve/idle control valve or one of those valves is faulty (you'll have one or t'other, not both), disconnected/split vacuum advance hose, fuel pressure regulator leaking air - could be many things!

Get an assistant to keep the throttle pressed where the engine hunts most and spray all round the top of the inlet manifold (where it joins the head) then the bottom, round the throttle body, round any hoses stubs on the inlet manifold, here there and everywhere until the engine note changes. Now narrows your search down to where exactly you have to spray if for the engine note to change.

Hunting is typically a lean mixture symptom. Do a proper diagnostic process with this in mind.

Can also be too rich causing eight stroking,in extreme cases.
Have you "read" the S/Plugs?
Good luck with your quest!

Regards,Keith.

Spray wd40 around the inlet manifold and vacuum hoses when running while someone has the throttle depressed slightly. If it briefly runs fine then there is a leak and it is sucking in too much air.

Thanks for adding the WD40 in - i forgot! ;) :D