Engine won't rev and hunts

[ANDTWENTY]

I just picked myself up a 2.3 1990 940 GLE non-turbo in absolute pristine condition, it has 1 previous owner and he had a full service history from the main stealer (not that this means much as the air filter that was changed 14 miles ago looks worse than the one on my car from a year ago).

The issue is that the car will only idle. I have seen some other posts about this but nothing conclusive. the car starts fine and idles great but as soon as you put on any accelerator the car will hunt and surge but will not rev up enough to drive. I feel it could be either the throttle position sensor or the maf sensor but after seeing the price id like to avoid that if I can. I tested the tps and it goes from 0v to 3.72v and thats it, it doesn't gradually go up and down so I believe this is bad, although the alternator has gone too so there is only 9.5v in the car so ill check it later with a charged battery.

The maf sensor is a 5 wire so I haven't checked this yet, any info in which cables to check would be greatly appreciated.

Is there anything else that it can be? someone said the idle air control valve which I will take out and clean as thats free. just trying to rule out the easy/free stuff before having to shell out on parts I don't need as I've been there before!

Thanks in advance for any help!!

[Clan]

Originally Posted by ANDTWENTY

I just picked myself up a 2.3 1990 940 GLE non-turbo in absolute pristine condition, it has 1 previous owner and he had a full service history from the main stealer (not that this means much as the air filter that was changed 14 miles ago looks worse than the one on my car from a year ago).

The issue is that the car will only idle. I have seen some other posts about this but nothing conclusive. the car starts fine and idles great but as soon as you put on any accelerator the car will hunt and surge but will not rev up enough to drive. I feel it could be either the throttle position sensor or the maf sensor but after seeing the price id like to avoid that if I can. I tested the tps and it goes from 0v to 3.72v and thats it, it doesn't gradually go up and down so I believe this is bad, although the alternator has gone too so there is only 9.5v in the car so ill check it later with a charged battery.

The maf sensor is a 5 wire so I haven't checked this yet, any info in which cables to check would be greatly appreciated.

Is there anything else that it can be? someone said the idle air control valve which I will take out and clean as thats free. just trying to rule out the easy/free stuff before having to shell out on parts I don't need as I've been there before!

Thanks in advance for any help!!

The most obvious thing is fuel pressure , easy to check with the test point valve on the fuel rail ...

[ANDTWENTY]

Originally Posted by Clan

The most obvious thing is fuel pressure , easy to check with the test point valve on the fuel rail ...

Thanks for the reply! How will I know if there is enough pressure, just if theres a good squirt of fuel or will I need a gauge?

[Clan]

Originally Posted by ANDTWENTY

Thanks for the reply! How will I know if there is enough pressure, just if theres a good squirt of fuel or will I need a gauge?

you will need a gauge , and decent garage should have one .. It should be 3.5 bar and stay the same whatever the engine load , it might be ok at idle but if it drops when you rev the engine hard that is your problem . I take it for granted you change the filter regularly !

[ANDTWENTY]

Originally Posted by Clan

you will need a gauge , and decent garage should have one .. It should be 3.5 bar and stay the same whatever the engine load , it might be ok at idle but if it drops when you rev the engine hard that is your problem . I take it for granted you change the filter regularly !

just had a look and there doesn't seem to be anywhere to test the pressure, no valve?

[Laird Scooby]

The most likely cause is a split in or a loose hose between the MAF and the throttle body. This will cause exactly the symptoms you're getting.

As for your tests on the TPS, what you've got seems right as it's not a sensor but two switches, one for idle and the other for full throttle.

However, you've told us the biggest fault of all. The alternator isn't giving an output and the battery is flat. Either of those will cause erratic running and can manifest itself in a number of ways.

First thing to do is to sort the alternator, you won't get any solid conclusions on the injection with a FUBAR alternator.

Does the charge warning light come one with the key at position 2? It could be something as simple as the bulb, to prove this find the thin red wire (NOT the thick one!) on the back of the alternator, unplug it and use a piece of wire to jump it to a good earth. With the key at position 2, does the charge warning light work? If not and the other warning lights come on (bulb failure, low washer, ABS if fitted and a few others) then it suggests the bulb has failed. If the bulb works but not when the red wire is reconnected to the alternator then the voltage regulator on the back of the alternator is suspect but there could be other problems in the alternator.

The choice of the next step depends very much what you find when you investigate the alternator. There are no short cuts, you need to fix the charging problem first. Don't forget to check the belt is tight!

[Clan]

Originally Posted by ANDTWENTY

just had a look and there doesn't seem to be anywhere to test the pressure, no valve?

the valve should be on the back end of the fuel rail which connects all the injectors together . ( The windscreen end ) usually with a blue cap . it is very much like a tyre air valve ..