940 having intermittent difficulty starting

[Laney760]

To be sure, I re-read the codes after a run today, the first run after the codes were cleared. I orignally had the engine coolant sensor code and the speedo missing signal code before the the engine coolant sensor was replaced. Now I only have the speedo missing signal code. So I know my code reader works properly and I know that I no doubt needed an engine coolant sensor! Next port of call definitely the crank sensor as suggested by Aardvark before I read the codes! Will update next week

[Dirty Rooster]

I had some odd non-starts for a few months : it was the alternator AND battery on their way out.
Not to suggest buying a pair but the Alt is very easy to check with a gizmo.

[Laney760]

Originally Posted by Laird Scooby

This might be helpful if you can't go to parts shops etc :

https://www.ebay.co.uk/itm/Crankshaf...4/372637210085

All similar prices for the same thing on ebay.

Cheers Dave, I've bought one of these

[Laney760]

Out of interest, do both log starting and petrol starting rely on the crank sensor. Car is definitely way better on warm starts, ie lpg. If they both rely on the crank sensor then I am suspicious of my 10 month old brand new Volvo fuel pump but surely a failing pump throws up a code?

[Laird Scooby]

Originally Posted by Laney760

Out of interest, do both log starting and petrol starting rely on the crank sensor. Car is definitely way better on warm starts, ie lpg. If they both rely on the crank sensor then I am suspicious of my 10 month old brand new Volvo fuel pump but surely a failing pump throws up a code?

Your car always starts on petrol Ellie - the temperature only makes a difference to how quickly it switches to LPG from petrol. If it's better on warm starts since renewing the CTS, that points to what i call a "lazy" sensor - not failed completely but has aged and deteriorated to the point where it's giving "out of range" data.

The sensor is a thermistor and they do deterioirate with age, for example if you have air-con with the climate control that maintains the temperature, that relies on a thermistor in the interior light to sense the cabin temperature. As the sensor gets older, the system thinks the cabin is cooler than it is so if you set it at say 22C, eventually it will actually give you 24C instead of 22C as it would have done from new.

Imagine that ten times worse and that would be a "lazy" CTS which would overfuel cold and hot. If it's better when it's hot now, you've cured that, the cold start is obviously better as you're getting the engine hot again!

With that in mind, if you consider the function of the CPS, everything else will make sense.
First and foremost, it sends a signal to the ignition ECU (mounted above and to the right of the accelerator if yours is the same as my 760 - i think it most likely is as the facelift 760 "paved the way" for the facelift 740 and the 940) to tell it to fire the ignition amplifier module (behind the left hand headlight on a heatsink) which in turn fires the coil.
Fairly sure you know the rest of the story on that, HT leads, distributor, plugs etc but the other thing the ignition ECU does is send a signal to the fuel ECU to tell it the engine is turning and at what speed.

When the fuel ECU gets this signal, it switches the fuel pump relay on (which in turn powers the pump(s) and Lambda sensor heating coil) and it starts firing the injectors.

Ash suggested it, i would have done as well if it hadn't been for the codes and you seem to have a hunch/gut-feeling about the CPS and to be fair, they are prone to failure on the red block engines.
Now you know the various tasks it performs and i know you said earlier in the thread you weren't sure you could hear the fuel pumps running for a second or two after trying to start it, it's a reasonably fair bet it's playing up.

When you change it, i'd be very interested in your observaations about how the car starts and runs, also on the general condition of the old one and its wiring.

I'm not saying any more than that for now as i don't want to influence any feelings/impressions/conclusions you may or may not have.

[Ian21401]

Thanks for the breakdown of what talks to what and what then is supposed to happen. I know that it’s basic but the mist is beginning to clear.

[Laney760]

Yes, thanks for the awesome post Dave! I was sending my partner, not a petrolhead, to sleep in the theatre last night when I read it to him in the interval!

I will update when the crank sensor is fitted

[Laney760]

I have to be quick because I had a flooded kitchen last night / this morning and I have to clear all the mess up before I have dinner and get to bed!

The part arrived on Thursday but I had such a busy weekend that I booked it into the garage for this quick job on my way home this evening. Garage removed the old crank sensor and judging by the state of it, will upload photos later, I thought no wonder there was a starting problem. You know what thought did!? They fitted the new crank sensor ---- same starting problem! Garage establised that there is fuel getting through and there is a spark, the latter they think eliminating the ignition amplifier? The garage checked the injectors but they are suspicious of the feed to the injectors and were talking about negative and postive feeds and injectors and also, I think I am quoting them correctly, they are wondering how does the gas system switch off the petrol ecu, where does the gas system cut in to do this, could this be the problem?

On the plus side I am pretty good at getting it started now and it's not such a big problem.

I am secretly hoping that when I go out in the morning she fires up easily thereby establishing that the crank sensor was the cause of the difficult cold starts and that there is a separate problem causing the hot start problem and that when the garage clean out the throttle body when I am on holiday in a few weeks time that that resolves the hot start issue? I am secretly hoping that it's a combo of the different things causing the cold and hot start problems, the morning will tell. For the record the throttle body was cleaned out about 3 years / 30 thou ago, too long ago, so it will need doing now and she had a new lambda probe at the same time. Maybe my next port of call with be the lambda sensor? I think the garage are suspicious of something to do with the injectors also, as have I been because on that first turn of the key she starts to fire, sometimes I can just catch it, but then she dies off straight away.

[Laney760]

One other thing? We had the petrol injectors unplugged from the injectors and she was running on gas, running well. I picked up one of the unplugged injector leads to take a look at it as it looked a bit ropey (the furthest one back) and that very second that I was handling it the engine stopped dead? Can't see any logic to this other than coincidence? Yet why she stop for no reason, she just doesn't do that.

[Laird Scooby]

If there's a big fat blue spark then the ignition system would appear to be OK Ellie, including the ignition amplifier module.

On the injection side, there is an ignition switched (through the RSR) +ve feed to the injectors, this remains whether running on petrol or gas.

On each injector there's also a -ve line, switched by the petrol ECU to fire the injector. On yours, you have what's known as "full-group" or "Batch-fire" injection, in other words, all 4 injectors fire at the same time.

On the -ve line to the injectors, the common practice is to cut into this with the LPG and the internal switching of the LPG ECU connects the cut wires back together for petrol operation. In LPG operation, it disconnects the petrol injector and uses a "dummy load" to emulate the petrol injector so the petrol ECU doesn't register a fault, uses the signal to trigger the timing for the LPG injection and uses separate outputs for the gas injectors.

However, if the garage is suggesting the -ve signal to the injectors is getting lost somewhere, this would effect starting on both petrol and gas. There was a thread on here recently where someone had a car that wouldn't start at all, eventually he used a piece of wire onto the -ve terminal of the injectors and flicked it to earth with the ignition on and it fired up - didn't run for long but proved he was in the right area. There was another problem or three as well but he didn't actually explain what he found.

With that in mind, i'll try and have a look at the diagrams later and see if there's a suspect point where the connections could go bad causing a problem for you.