Throttle control

[davekit]

I own a 1987 Volvo 480 with B18E engine. The car appears to have a genuine mileage of only 18k, backed by MoTs and receipts, and lack of rust, etc. I have eliminated many problems, mostly electrical, but the engine seemed sluggish, and recently developed a bad misfire once warmed up. Various things have been tried to sort this (with help from fellow owners on here), but to no avail.

Different air and water temperature sensors have been tried, and a new coil, but no improvement. More recently I had the injectors cleaned, again no joy. I sent the ECU away for testing but that was working perfectly. Finally I took the car to The Motorist in Sherburn-in-Elmet to see if they could find the fault.

Initially they tried a smoke test - removing the air filter and blowing smoke into the air inlet with the engine ticking over. This showed there were air leaks around the temp sensor (possibly as a result of removing it several times), and around the throttle control valve (seating and spindle). It should be possible to seal the temp sensor and the seating of the control valve, but the spindle wear suggests the valve needs replacing or repairing. New valves seem to be unobtainable, and secondhand ones will likely have done many more miles than this one. Does anyone know of someone who can rebuild the valve to replace bushes and so on?

A second related problem has now been found, which may be easier to fix, and may prove to be the real cause of the bad running. The throttle position sensor is only reading 'on' or 'off', instead of a range of values. I assume it works on some sort of rheostat. Has anyone got any experience with replacing these? I found some on ebay which would "fit my vehicle" as they say. The listing numbers of a couple of likely ones are 394214977063 and 285808509171, but as the car is elsewhere I can't visually compare them to the current fitment, and the part numbers do not match those in the partsbook. Any help or guidance appreciated, before I get back to The Motorist.

Thanks - Dave

[jamescarruthers]

Hey Dave,

Throttle position sensor really does read just on and off on your engine. It tells the ECU to use the idle air control valve to maintain idle and that’s about it. You should hear it click with the lightest pull of the throttle cable. If not, it needs adjusted by moving on the slotted holes.

How were the CO emissions on the MOT? Is the potentiometer set up to give the right CO at idle.

The throttle body is a double intake Webber I think so nothing particularly bespoke to the 480. I have no idea where to get the bits to make it all tight again, but would also be interested if you do work it out.

[jifflemon]

Also remember that the Volume of air entering the engine is measured by the Map sensor, AFTER the throttle body.

As James has already mentioned, the throttle position sensor measures only 2 things: When the throttle is closed (and thus, idle) or when the throttle is wide open.

In terms of misfire, have you isolated down to a single cylinder or all cylinders?
Cap/rotor/leads/Plugs all checked?
Compression check when cold and hot?

[davekit]

Hi James, Jeff
Thanks for your comments, I have passed them on to The Motorist.

CO emissions were spot on at the last MoT. I have played with the potentiometer, but ended up returning it to its original setting.
I always associate Weber with carburettors, but a quick search showed they do a bit more. Website webcon.co.uk - only one Volvo throttle assembly listed, no idea if suitable (car elsewhere so can't even do a visual comparison) and nearly £500 plus VAT. Also found a linkage kit for Volvo B18/B20 DGV at just over £100 plus VAT, but whether it contains the right parts, I couldn't say.

I may be confused over the various sensors, but I thought MAP (and MAF) sensors were only on turbo engines. Please tell me I'm wrong! Mine is non turbo.
The misfire is so violent that it must be affecting all cylinders. If only on one, I'd expect the lack of power, but not such a bad misfire.
All service items have been renewed - cap, rotor, leads, plugs, filters, cam belt. In the latter case, I know that if the belt is out of position, it can have disasterous consequences. But if only say one tooth out, would the engine still run? The lack of power was there before the belt change but the old belt was clearly not the original, so had been changed before. (The misfire crept in much later, so not due to the belt change).
Compression has not been checked.

We battle on! Dave

[jamescarruthers]

Turbo cars all have a MAF (manifold air flow) sensor to work out how much air is getting in to the engine. They all have pretty much the same engine management system.

Early N/A cars have a MAP (manifold air pressure) sensor and an intake air temperature sensor to work out how much air is getting in to the engine.

Later N/A cars changed to a set up similar to the Turbo cars. Later 1.7 and all 2.0s. A certain version of Feniz introduced but I can’t remember off the top of my head. (3.2?).

The difference that the potentiometer can make in fuelling CO emissions is quite huge. I recently got to tune mine at the MOT station and the acceptable range is all in about 3 degrees of a turn!