Peter Zimmermann

JBO: Oh my, I would never use one of those things. From my point of view they fall into the category of "poor" aftermarket items. When the factory redesigned the airbox, about 1980, and inserted the cold start fuel difusser inside the central chamber of the airbox that fixed the problem. Airboxes rarely blow on properly maintained cars because air leaks are controlled, plug wire condition is monitored, cold control pressure is within spec and fuel mixture is set properly at every tuneup, and kept in spec. I will cut those with '73.5 - 75 cars, with a hand throttle, a little slack. The hand throttle is a tricky adjustment that requires that the throttle body butterfly opens 7 degrees. If it opens more, the car will be too lean for proper cold starting. Mechanical parts in the throttle linkage can wear, plus the basic hand throttle adjustment itself is often misunderstood, so the airbox has more opportunities to blow in those cars. By the way, replacement airboxes for '74/75 cars get the new internal diffuser, but the '73.5 box doesn't. OK, the automatic cold start system used from '76 was wonderful, and got better almost every year. It is the mechanic's responsibility to advise every owner regarding cold control pressure, but it's often overlooked. If that pressure is set to lean, it can shorten airbox life expectancy. Air leaks, also the cold start enemy (as the engine warms parts expand and hide the symptoms) mean that the car's owner must be in tune with his car and report cold start difficulty, such as small backfires. Every 15K miles the car should be professionally serviced, which includes a two minute flashlight inspection of the intake manifold connecting sleeves and their clamps, as well as a wiggle of each injector. Also, the CO% (mixture) must be checked and set as necessary at every 15K mile major maintenance. That's crucial to airbox life and can also tell your mechanic well in advance of a problem that something is changing. Here's the kicker. I've had cars towed in to my shop with blown airboxes (neatly fitted WITH a pop-off valve), I've had cars brought in that had faulty pop-off valve seals (which cause high rpm lean conditions and increased oil temp), I've had cars towed in with pop-off valves stuck open causing a condition that was so lean the car could only crank, and I've had cars towed in where the epoxy failed and the valve was dislodged. For the record, my shop never installed a pop-off valve, unless it was one dislodged that was installed by others. I believe that a pop-off valve has the ability to save a box now and then, but if it "pops" there is a problem with the car that should be corrected. Otherwise, the box will blow anyway. There is probably another justifiable situation to use one, and that's if you love your car, but there is no quality available service within a reasonable distance to where you live. Maybe then, maybe, I might think using one is OK. But even with it, if the car has bad plug wires, or unmetered air entering, or the cold control pressure is wrong, or the mixture isn't right, the box is going to blow anyway. Oops, I think that this thread has been hijacked... Sorry!
Pete

Edward

Wow Peter, that was ummm, detailed
Thanks, though; I could really use that kind of info. My SC is running superbly, but CIS is new to me. Any reference you could recommend for to check and set all that you mentioned for when "that time" comes? TIA!

Graham,
"Hunting" for a few seconds on DME cars is pretty harmless. But what I've found useful is a tankful (or two) with a good fuel additive and some spirited driving ends the hunting altogether (in my experience, anyway). I've used Techron with good results, but have now gone to LubroMoly's VentilSauber and Jectron, their two additives for cleaning valve areas and the fuel system. Once I did these treatments (2-3 per year), my hunt went away and has never returned.

Edward

Steve W

Although idle hunt is common, it should not be tolerated as acceptable. The engine's idle is controlled by an idle control valve motor, and it only has a narrow margin where it can comfortably operate. If where the engine want to naturally idle is out of this operating margin, this idle control motor goes wack trying to bring the rpm to target, oscillating out of sync with the rise and fall of the engine speed. A cold start idle hunt often times means the base idle mixture is off. It stops after 15 seconds or so, because the O2 sensor heats up and kicks in regulating the mixture back to stoichiometric. A mixture richer than stoichiometric will idle high, and vice versa. You can adjust the base idle mixture via the mixture screw under the air flow meter, however, make sure the idle mixture is not off because of intake air leaks. Those should be corrected first. Shrinking intake manifold gaskets are a common cause. Engine starting fluid (ether) makes a great leak detector.

After adjusting the base idle speed, if a hunt still occurs, the base idle speed will probably also need to be set. This is done with the idle speed screw on the throttle body. Again, you want to bring the base speed within the operating margin of the idle control valve so it's job and influence is minimal.

Peter Zimmermann

Steve: Well said. I would like to add that the factory manual is strict and specific regarding setting the idle speed (engine oil temperature, etc.) and finally, fuel mixture. The process is not something that the average DIYer can do at home (a precise exhaust gas analyzer is required). The intake manifold gaskets are a very good point. During replacement of the fuel distribution line on top of the engine (3.2 cars) the manifold must be removed. We saw the poor condition of the gaskets and knew immediately that resetting the CO% after everything was back together must be part of that job.
Pete

Edward

Peter,

So a DIY-er with a Bentley is not enough ...I'd need a gas analyzer to set mixture accurately? How does the CIS' mixture actually change over time? I can understand gasket, manifold, or vaccuum leaks developing over time, but how the CIS? TIA

Edward

Peter Zimmermann

Edward: Unfortunately, with CIS, almost everything that can leak air is downstream from the mixture unit, hence, almost everything can affect mixture. The injectors are sealed with an o-ring, and are pressed into a plastic sleeve that seals to the manifold with an o-ring. Over time, all 12 of those o-rings will begin to seep air, leaning out the mixture. The same goes for the 6 connecting sleeves between the airbox and intake runners, they begin to leak air. Remember, it doesn't take much. Now, let's move upstream to the sensor plate and its housing. The sensor plate has a specific setting, and vibration over many miles, even tiny backfires that you're not aware of, can alter it, which will impact the mixture. The sensor plate housing/throttle body boot should be removed at least every other major service, and the funnel of the housing cleaned thoroughly. In cases when the car is run on crummy gas (anything but Chevron!) gummy crud can build up in and under the fuel distributor, which can affect mixture. Finally, when the valves are adjusted, the change in intake manifold vacuum that results will impact the mixture, which is the reason that the CO% adjustment is a crucial part of every major service. Of course, parts like the warm up regulator, and the cold start system tubing, among others, can fail, which will all lead to mixture changes. With all this in mind, the CIS system, especially post-Lambda, is an extremely stable system, and many times the CO% doesn't change between services. But if it does, you want to know about it!
Pete

450knotOffice

Mine's an '84 and hunts a little too when it's cold and hasn't run for a few days or more. What it does is this: It'll light off in a second or two and then settle into a rough idle at about 800 RPM for about 3 seconds, then it'll start to pick up the RPM, slowly increasing for about 2 to 3 seconds until it hits about 1200, at which point it'll drop suddenly to about 800 but smooth out, then slowly pick up again to about 950 or so. After this short little episode, which lasts about 10 to 15 seconds total, everything will be nice and smooth.

The car has always done this but it doesn't seem to be a problem.

GMWTUK

Hi Everyone,
As promised, here is an up-date. Thanks for all of the advice, much appreciated
Having established that there were no leaks and also that the settings were correct, I dumped a can of wynnes fuel additive into a full tank and after a while I noticed an improvement, although only slight. More regular use has certainly helped, but I think the fuel I was using was not helping either. Here in the UK we have regular unleaded fuel 95.RON, and also super unleaded 98.RON which is more expensive and most people think is a total con as most cars are set to run on 95.RON anyway. Then we have Shell Optimax which is 99RON and everyone thinks this is the ultimate con as its even more expensive. I took my daily drive in for a service and asked the engineer if it would make any difference using the different fuels and he said no as the car has a sufficiently sophisticated brain to make a zillion adjustments a second and it's set for 95RON. Thats an 03 BMW X5 !
What about an 89 Porsche? I asked.
Its the only way to go he said, Super or Optimax!.
Did it and bugger me he was right..it makes a difference. Smooth starting and running and a steady tickover
I don't know it was a combination of all three but I will use Optimax from now on despite our rapacious fuel prices here in the UK, but hey, at least we have a decent justice system here...I lost £10.00 on the MJ verdict!

AJ88CAB

Graham

Glad to hear your problem has gone away. If it comes back you might take a look at your idle switch as well. I had a hunting problem a few months back and initially found a loose vacuum hose which fixed it for awhile. Then it started again and I tracked it down to the idle switch. Two unrelated problems but basically the same symptom.

Good Luck,
Andrew