200FT Hard cold start. Pulls like a train when warm.

[Danders]

Originally Posted by AndyV7o

Updates are always good 👍 I hate unfinished stories, of which forums have many!

Particularly on threads regarding starting issues on old Volvos!

[Laird Scooby]

Originally Posted by Danders

Of course I excitedly retested the resistance of the two sensors at the ECU and found that they read the same.... mostly. What I had previously put down to my cheap knock off Fluke meter causing spurious readings when measuring resistance, looks like actually the new (fitted) sensor was giving spurious readings. By spurious I mean open circuit, then a second of recognisable resistance, then open circuit then various other resiatcnes in between. I guess I read what i wanted to see-the good reading and doubted my tool instead. Bad workmen eh?

I get a solid reading from the ‘old’ sensor.

Would that be the RS "Isotech" Fluke knock-off by any chance? If so, they're every bit as good as Fluke originals and Fluke will even calibrate them - or at least they would last time i checked!

Remember sometime back in this thread i commented to someone that no matter what the fault is, the evidence has been there from the start? I genuinely thought you'd already tried refitting the "old" sensor as well.

The bit that caused the problem was that you changed the dizzy as well - gave yourself two possibles as being the cause of the non-start. Old engineering rule of thumb when playing "parts-substitution" diagnosis is if adding a different part doesn't fix it, refit the original so as not to confuse the situation.

I can only summise that with the dodgy signal from the new sensor, the ECU first saw a cold engine (with resistance) then a hot engine (lower resistance) then a failed sensor (open circuit) that then fixed itself but reported "hot".
The older ECUs aren't sophisticated enough to implement a strategy whereby it would try a few different fueling options until the engine started, not sure even that the new ones are!

It would seem you've found the fault though! Fingers crossed!

[Danders]

To draw a line under this; the car is sweet now, running fine.

Temperature sensor was the problem - it must have been deteriorating in some way which caused incorrect fuelling, but could be trimmed out by the lambda when warm.

If you have similar problems, check the resistance across each pin on the temp sensor, each individually to ground.

I found its easily replaced with coolant still in, if you're quick. Also, it helps if you have a deep socket to reach over the eceltrical connector(I used a sparkplug socket with the rubber grommet removed).

Thanks again for your help all.

Andy

[Laird Scooby]

Glad it's all sorted for you Andy - why did you change the CTS in the first place?

[Tyron760]

Originally Posted by Danders

And the dodgy resistance readings are only on one pin of the ‘ new’ sensor. So I suspect the thermistor(?) on the pin, rather than the ground.

Please name and shame! What make was the new dodgy CTS? So that I and maybe others could avoid same.

Glad you got it sorted, wish you many trouble free miles!

[Stephen Edwin]

Originally Posted by Tyron760

Please name and shame! What make was the new dodgy CTS? So that I and maybe others could avoid same.

Glad you got it sorted, wish you many trouble free miles!

In fairness, one non-working item does not condemn the entire production from that supplier. Other suppliers might have a higher percentage of failures but we just haven't heard about them? Although admittedly it would make me cautious of that supplier .... depending on their usual reputation.

Whilst posting, I say credit to Dave for posting what one should expect re siberia type info in different scenarios. The CTS was already suspect in my ignorant thoughts because disconnecting it allowed the engine to start. But that info from Dave sort of underlined the point. That CTS failed differently from usual failures....moral is be prepared for the unexpected.

Above all .... great that it is sorted Andy. Cushti.


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[Danders]

Hi all. Was an ebay special. Item 283051494976.

The reason for changing in the first place (seems so long ago..) was just after some googling and forum reading. I didn't actually test the old sensor before just popping the new in. Lesson learnt. It was the parts cannon approach early on.

It was always a hunch also that it was running too rich - when I pulled the plugs they were clearly wet with fuel and I was pretty confident spark was good. Temperature sensor, for a few quid seemed like a good place to start, especially as others had seen issues with it, with similar symptoms.

[Laird Scooby]

That's a Parts Monster own brand special - normally they dish out pretty good stuff so i'd say you were just unlucky this time.

Also unlucky in your diagnosis, for the same reason medical doctors don't like people "googling" symptoms of illnesses online as very often they come up with a worst case scenario, it seems you may have done the same and instead of curing the problem, made it worse.

Engines are designed to run rich when cold, there's no Lambda feedback as the sensors don't work until they reach a certain temperature so it's somewhat arbitrary enrichment - this can also lead to misdiagnosis by reading the plugs in a cold engine.

Judging whether a spark is good can also be difficult. If it jumps 1/2" in free air you've got at least 25kV which should be enough to jump 0.6mm in an engine where the pressure is up to about 160-180psi but it needs to be blue in colour to have some strength behind it.

If it's more orange and only jumping say 1/4", you're down to about 5-6kV with not much grunt behind it - unlikely to "make the journey" across the plug gap.