Please help a noob - 924 cold running issues

Hi all, I was directed to this forum by a fellow dubber who suggested there were some very learned K-Jet folks on here that might be able to help me diagnose and test for a running issue on my 924 2.0 NA K-Jet.

When starting from cold, it fires instantly and will run without cutting out, but runs very roughly at 950-1,000 rpm as if it desperately needs a fast idle and some fuel enrichment. If I bring the revs up to 2,000 rpm, it's fine and smooths out, but as soon as I lift off, it struggles along with a rough and lumpy idle until warm.

After a few minutes of warming up, it runs perfectly and hot starting is also perfect.

I know little about the CIS system, although initial searching suggested I should check the auxiliary air valve first. I removed it and when cold can see through it and after a warm up under a hair dryer, it closes up. I assume therefore this is not the culprit.

Am I correct is concluding that because it fires easily when cold, and because my cold running issues are prolonged until the engine starts to warm up that it is unlikely to be the cold start valve which if I understand correctly squirts additional fuel for between 2 and 12 seconds to aide cold starting. As it starts easily and quickly, can I rule out the CSV?

At this point I'm stuck and need help to diagnose and test for the problem.

What is the warm up regulator I hear about? What is it;s function, how does it work, how is it controlled and how do I test it? Is the WUR a possible cause?

Could do with some guidance.

Many thanks in advance.

 

Sadly, I'm lacking a pressure gauge or workshop manual. I only have the next to useless Haynes.

Is there a simple bench test I can do?

I found this.... http://b2resource.com/PDFs/wurfix.pdf which if I understanding it correctly says that as the electrical element heats up the bi-metallic strip along with engine heat, it pulls away from the spring to increase fuel pressure when warm. Therefore when cold, the bi-metallic strip pushes against the spring to reduce fuel control pressure to allow more fuel to the injectors when cold?

Am I correct in thinking that the function of the electrics wouldn't effect the cold running, as the electrics (and engine heat) are there to increase pressure when warm?

If that is the case, how does a failed unit effect cold running if it's natural state is pushing down on the bring to reduce fuel pressure? Stuck spring? Broken or damaged bi-metallic strip?

Am I on the right track?

 

That's what I thought. I can imagine them failing and giving warm running issues, but it;s natural state is cold, so for it to fail when cold would indicate the the bi-metallic strip or spring was goosed which would then effect warm running - which is perfect.

Could it be anything else?

Are my conclusions about the cold start valve and aux air regulator correct?

 

Yep, checked the aux air valve and working fine.

Can I just clarify - if the purpose of the heating element is to bend the strip away from the spring, hence increasing pressure when warm, then for cold running issues, the be-metallic strip whoudl be acting upon the spring to reduce pressure?

If this is the case, then a faulty heating element wouldn't change the cold running status? it would only delay the increase of fuel pressure until the engine warmed the unit up sufficiently?

I'm just trying to see how a faulty WUR could alter cold running only? When cold, it doesn't actually do anything? Only when warm does it increase pressure be releasing it's effect on the spring.

Have I got that right?

 

I'm trying to remove the WUR to clean test and inspect it but how and where to I isolate the fuel. The larger 14mm banjo is fine it just leaked a small amount of fuel as I undid it, bhowever the smaller 13mm banjo just endlessly leaks fuel when removed. I gave up after two mug fulls on the basis that I felt it would carry on until it drained the tank. I traced this line back to the fuel distribution block where it fed into a banjo that came out the side with two lines feeding into the same bango - and removing that one leaked heavily.

Where how do I isolate the fuel to this line?

All the line seem hard and not easily clamped.

 

I removed the rear jack stands and with the car nose up, the fuel flow ceased, allowing me to remove the WUR and test it. Simple case of gravity causing draining from the fuel tank return line.

On the bench, it measured 19 ohms. The Haynes manual says it should read 20 ohms, so I was happy that the electric heating coil hadn't failed.

I tested the spare one and it measured 19 ohms also, so I was happy that I had two WURs with working heating elements.

Therefore if there was a fault, it was either mechanical or a blockage on the ports.

For the record, the ports were spotless.

At this point I thought it was worth just fitting the spare unit to see if it made a difference to how it ran from cold.

And boy, what a difference - it now hardly runs at all.

It will fire, but won't run. If I kick the throttle hard a few times I can get the revs up, but it's spluttering, coughing and popping and won't idle at all. Tried several times - no difference.

So, I've either disturbed something making it run like crap now, or the WUR I've just fitted really is knackered, and the one I suspected was faulty, actually isn't and the lack of fast idle from cold is something else.

Or the one I took off is still faulty, but not as faulty as the one I replaced it with.

Do I refit the one that was known to at least run and idle, albeit roughly from cold until warm, and if it then runs as it did before hand, I know the spare one I fitted is duff and I've not messed up anything else?

So far we know....

1) The cold start injector works as it fires instantly.

2) The aux air valve works - removed and tested, hot and cold.

3) With the original WUR unit, it ran perfectly when warm, but needed a fast idle when started from cold, but would at least idle.

Any thoughts?

The thing that is bugging me is that when warm, it drives perfectly - no flat spots, no hesitation, no missing or coughing, doesn't run rich, clear exhaust - all as it should be which tells me that fundamentally the fuel and ignition system is sound.

If it's only missing a fast idle from cold, that suggests that it is something to do with a supplementary system failure, designed to aid cold start up, of which there are three - aux air, cold start injector and the WUR, two of which we know are working.

 

Do you have an Idle Stabilization Valve / System and can you identify the CIS related engine / coolant temperature sensors ?

 

No, just the aux air, WUR and cold start injector. There's no real electronics to it.

For the record the in tank fuel pump has been disconnected and the car just uses the normal in line pump. They used to only have one pump anyway, but could that be an issue? Having said that, it runs perfectly when warm and at high revs/fuel demand....

 

Tonight's experiment...

I stripped down the WUR that was native to the car to see what state it was inside. So far we know that the heating coil is fine as it measured 19 ohms.

I stripped it down to find this...







It was absolutely pristine and the ports where clean and clear as a whistle...



I blew through them with zero resistance and cleaned them with brake cleaner just to be sure and then put it back together.

Whilst doing so I noticed that the brass boss that the bi-metallic strip is mounted to has taken some hits from a punch in the past which I guess was someone's attempt to adjust the fuel pressure previously?



This set alarm bells ringing, so I used my digital verniers to measure the depth of the brass plug on the spare WUR I fitted without success last night, knowing it was virginal and had not been touched due to having a light unmarked patina on the surface of the brass plug. Then with some gentle taps using a hammer and drift, I reset the one I had stripped to exact same depth as the spare unit. Whilst not perfectly accurate, it at least tells me that the unit is now as near as possible to a factory unit, with a working coil and immaculate internals.

As I'm now well practiced in removing the until, I put it all back together and tried again. It's certainly better than it was before I started to investigate, but it still idles at 900 rpm cold, which just feels too slow and hence there is vibration and roughness.

I've taken a short video of the first cold start after reassembly with no fuel in the lines, hence it take a couple of cranks to fire, but settles quickly. The second half of the video is a hot start taken 20 mins later after I'd left it running. Fires straight up and sounds great. So I now have a car that fires and runs instantly from hot and cold, but just idles too slowly when cold.

[youtube]mtB8yR8o1xg[/youtube]

So, what controls the cold idle speed on the K-Jetronic?