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Hey Guys, I hope You don't mind me popping in here, Im actually working on a 924 turbo, and I am on the 924 forum, but it's not quite as active.
Since the early 928s use CIS, I hope you all might be another avenue of knowledge.
So, I've been tuning the CIS to get the AFR's right, especially under boost. And I was getting them almost where I wanted them on my last test drive, when all of a sudden it was idling poorly and barely had any power from a take off/hesitating on initial tip in. But once it got moving and got some rpms it got a little better.
I can't seem to find the issue yet. Now I have both a AFR and inline CIS pressure gauge, Along with boost gauge and an adjustable WUR.
At the moment the AFRs are bouncing and the engine is missing, along with the hesitation when you crack the throttle.
I assumed it was a Vac leak. But I can't find one. I have previously replaced most hoses, some O-rings and rubber elbows. Checked the sparkplugs for tightness and hose clamps are tight.
I used Starting fluid, Propane and soapy water, and nothing showed up. I was at first leaning towards intake gaskets, but last time I had the intake off I used some RTV on them, and again the test methods didn't show anything)
I looked very carefully at all the lines and hoses and just can't find anything, and I just rebuilt the Throttle Body.
But, one thing that I just noticed is that, when I started it cold, it seemed ok for the first 3-4 min, with steady AFR and no missing even with opening the throttle, and then it started again :-(
So I'm trying to think, what would change in those first few minutes? I realize the engine is heating up, but what would that affect??
And since it does run ok for those first few minutes, what can I rule out? I was afraid I burned a valve or piston (but then this is stock and only running 6psi boost)
thanks for your input.
Last edited by Not_Sure; Mar 14, 2021 at 07:08 PM.
While I am by no means a CIS guru, I would think that it runs normally during the initial few minutes and then changes would point to it being warmup related.
If the thermo-vacuum switch kicked over, the extra emissions stuff plumbed to it could be contributing to the fun, as well as the auxiliary fuel injector turning off, if that temp switch had just reached it's trigger temp.
Another possibility would be if your tuning adjustments were settings intended for the engine when fully warm, but you have it set to achieve the same condition when cold, since the auxiliary air valve and the cold start injector again play a significant role in cold start of a CIS managed engine.
All that said, it's speculation and guesswork on my part, and likely someone with actual experience tuning CIS will chime in.
the only thing that make changes with heat is the WUR.... did you check the control pressure it gives cold/warm ?
Yes thats one of the things I have been fine tuning. I had just dialed it in on my last test drive when this started. With the permanent CIS gauge, I would drive, Monitor AFR, get out and adjust, Drive again.
While I am by no means a CIS guru, I would think that it runs normally during the initial few minutes and then changes would point to it being warmup related.
If the thermo-vacuum switch kicked over, the extra emissions stuff plumbed to it could be contributing to the fun, as well as the auxiliary fuel injector turning off, if that temp switch had just reached it's trigger temp.
Another possibility would be if your tuning adjustments were settings intended for the engine when fully warm, but you have it set to achieve the same condition when cold, since the auxiliary air valve and the cold start injector again play a significant role in cold start of a CIS managed engine.
All that said, it's speculation and guesswork on my part, and likely someone with actual experience tuning CIS will chime in.
That all would be reasonable to think, But The engine was running normally during the last test drive, it started fine and ran fine and was fully warmed up, when this "all of a sudden" happened. Basically I gave it a boosted run, from one stopsign to the next, then from a stop i tried to take off again, and it starts bogging, at if something blew.
I've been playing with the CIS/WUR for the past several weeks including taking it apart several times, it seemed to be worked well, especially with the extra adjustment capability.
The cold start injector only runs 5-10 seconds.
So thats where it throws me, If something "blew" well...why isnt it showing up when the engine is cold?
Yes thats one of the things I have been fine tuning. I had just dialed it in on my last test drive when this started. With the permanent CIS gauge, I would drive, Monitor AFR, get out and adjust, Drive again.
something wrong with the FD? if system and control pressures are OK , even with temp .... what's left ?
Or a mechanical fault ? less chance ....
Im just debating where to start digging deeper. I figure if it runs ok at cold start, that probably means the valves and pistons are ok right? I thought about the possibily of a cracked head or HG, but the coolant/oil look good.
Thanks, I already have the factory diagrams and pressures. Keep in mind I can easily adjust the Control pressure (and mixture on the CIS) and that makes no difference in the missing.
I guess the question is; What could have failed while driving THAT wouldn't be affected at cold start? I am leaning away from a Fuel issue.
CIS certainly works good until it doesn't. Heh... I've fought with it for a while myself.
My first inclination was the same thing you had, a vacuum leak. If you've got a propane torch then turn it on and shoot some propane gas around the engine bay/hoses to see if the idle changes. If it does, that's where you can find a leak.
Maybe also cut down the number of "extra" vacuum lines temporarily to see if that effects the issue.
If you get no luck after that... Then I'd be suspicious of the warm up regulator (WUR) and it's lines. Because you said you've been "working on it" I wonder if a piece of dust/junk got into the WUR or one of it's lines.
This could certainly effect the running conditions cold -VS- hot.
Unless you're going about this all wrong and maybe you've got a new ignition problem. Ignition advance changes from cold to hot as well. Maybe even a weak coil. This would also cause popping and AFR to go crazy.
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CIS running rough
