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Old 06-30-2009, 08:13 PM
WANT '71 280SEL's Avatar
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M117 4.5 vacuum leak and cold start question

First of all, it's good to be back here. My brother wrecked my S320 and I haven't had the time or money to make progress on my '66 230S. As some of you may remember, I also had a '79 300SD and an '87 300SDL, all of which have been sold.

I've just recently (two days ago) acquired a '72 280SEL 4.5. I got it running yesterday. The car idles very high. I didn't put a tach on it, but I'd say 1500 rpm's or so. There is a large sucking noise (vacuum leak) which seems to be coming from the front of the intake. We sprayed carb cleaner and sure enough, there seems to be a vacuum leak, but it looks like it's all along the intake! The worst is definitely at the front. To top it off, the front two cylinders aren't firing at all. We disconnected the spark plus wires and nothing happened as did with every other cylinder. It is a very pronounced "hissing" noise. In addition, the car is running very rich. So, to sum it up, the car is running rich (even when warmed to running temp), idles at about 1500 rpm's, on only 6 cylinders. Needless to say, I'd imagine the car has run better in the past.

Could a vacuum leak be large enough to keep the front 2 cylinders from firing at all? I'm not sure what the big pipes are with rubber hoses that attach to a "block" type piece where the cold start mechanism is. My uncle and I are inclined to think the car is stuck in cold start mode. Disconnecting the two wire plug from what we're pretty sure is the cold start valve, does nothing, hot or cold. The car takes about 5-10 seconds of cranking to start, hot or cold. Yesterday, when I got it running after sitting for 2 years, it fired up within 2 seconds of cranking, who knows...

Sorry to ramble on so much. Obviously I need to dive in and do the intake gasket and associated seals (plenum donuts, and such), but what's with the 2 metal pipes that screw into the lower half of the intake? I'm not sure how the cold start mechanism works on this car.

Thanks,
David

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Old 06-30-2009, 08:24 PM
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Those 2 "Pipes" are for the idle air circuit. The cold start valve on yours sounds like it's not going on at all, hence needing the 10 seconds of cold cranking. My 4.5 hasn't been fired in 6 months or more but I bet if I tried, it'd start in under 3 seconds (although I disconnect the trigger point plug until oil pressure builds to 45PSI on starts after sitting that long).

It sounds like you have several issues. First of all, you will hear a large hissing sound with the air filter disconnected, as there is a hole in the top of the throttle body above the throttle plate that feeds your idle air circuit - and you can plug it and your car will stall. If it doesn't, your throttle plate needs to be adjusted, and/or you have massive vacuum leaks. You should replace the vacuum hose to the MAP sensor. Also, for troubleshooting, plug the line going to the door locks so that isn't bleeding vacuum out of your system.

If you have leaks all along the intake, you need new plenum seals. You'll need 8 of them, and while doing them, you'll need to replace both intake manifold gaskets as well because they will be destroyed when you remove the manifold. Although you should replace all these before diagnosing any misfire, you should do a simple compression test to make sure there are no burned valves or shot cylinders on the 2 affected cyls before pouring money into it.

If the compression test comes back fine, replace your manifold & plenum seals, and while you have the manifold off, send the 8 injectors out to get cleaned & tested. You may have 2 dead injectors (or at least completely clogged) causing the misfires. You may have a bad set of trigger points, or a bad ECU ground, or a bad ECU if 1+5 are the missing cylinders.
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Old 06-30-2009, 08:31 PM
WANT '71 280SEL's Avatar
I'll Go Upside Your Head!
 
Join Date: Jan 2003
Location: Dallas, TX
Posts: 3,378
Thanks for the input, Tomguy. I forgot to mention, I plugged the throttle body with my hand and the car hardly slowed down. So there is a huge vacuum leak, but would this cause the car to idle really high and run rich? There is definitely spark being sent TO the cylinders. I checked it at the end of the plug wire. Good advice on the compression check, I'll do that when I get back in town.

Thanks,
David
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Old 06-30-2009, 09:32 PM
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Quote:
Originally Posted by WANT '71 280SEL View Post
So there is a huge vacuum leak, but would this cause the car to idle really high and run rich?
Yes. The MAP sensor richens the mixture when it determines the need. The need is based off of manifold vacuum. Idle and just cruising, vacuum in the manifold is high, so it will not give the engine much gas. When you put your foot in it, it opens the throttle body and the manifold vacuum is reduced substantially, the MAP determines the engine is getting more air, so it dumps in more fuel.

Your throttle position switch is misaligned too, your car should be "Hunting" - high idle, low idle, high idle, low idle. Your idle circuit isn't being detected so your engine is in "Runaway idle".

If covering the throttle body completely did not kill the engine, you have bad vacuum leaks. If covering it completely barely slowed your runaway idle, don't run the engine like that. It's WAY too rich, and your oil is probably the viscosity of boiling water due to gas contamination. Fix the leaks and change your oil.

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2018 Durango R/T

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1999 Chrysler 300M - RIP @ 221k
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