450sl cold and hot high idle

[blk914]

I have a 1972 450sl, 144,000 miles, has been running perfect up till the other day. Coming home from work it developed a high idle problem, around 2000rpm hot. Cold starts it goes up to 3000rpm +. As far as I can tell no vacuum leaks (plenum and intake gaskets, injector seals changed about a year ago with new vacuum lines) no stuck linkage problems. Took AAV apart, cleaned, not sticking , checked element, OK. Is there anything else to check? Would a cold start valve cause this? Really need some help on this one. Appreciate any and all input, thanks.

[meltedpanda]

not likely, that should only fire at start up , if it continued to run , it would run rich but doubt rpms would increase.

sounds like you eliminated vacuum leaks, whcih would be my first thought. Did you check all the lines to injectors, rails and such? Try pluging and unpluging the air cleaner sensor when cold , if the idle does not increase that could be an issue. make sure your distributor is tight, make sure the vaccuum pull for the advance is there and plate is not stuck inside the dizzy. Your TPS could also be the issue, eliminate these first and let us know

BTW mine is a 72, VIN has it at 651 st ever built, is yours and early one?

[meltedpanda]

Curious. You didn't just change fueL pump did you?
Check for a crack in intake

[blk914]

Thanks for the reply Panda. I really think I've got a vacuum leak somewhere, fully warmed up I can plug the inlet in the throttle body to the AAV and the idle comes down to somewhat normal, I still think that's OK and plugging it is compensating for a leak somewhere else, it has to be since the cold start rpm is so high. I compared it to another AAV and the cold opening is the same. Evidently these cars are very sensitive to vacuum and I haven't pinpointed a leak yet. Other than gas mileage( around 10 in town ) the car has run outstanding and I'm trying not to disturb too much at this point. I haven't changed the fuel pump and hopefully manifold is not cracked, I really don't want to pull that back off any time soon to check. BTW # 004493 on the VIN.
Thanks alot,
Jim

(please disregard an earlier reply that I deleted if you happened to read, thanks)

[Graham]

Originally Posted by blk914

Thanks for the reply Panda. I really think I've got a vacuum leak somewhere, fully warmed up I can plug the inlet in the throttle body to the AAV and the idle comes down to somewhat normal, I still think that's OK and plugging it is compensating for a leak somewhere else, it has to be since the cold start rpm is so high.

When you block the line at the throttle body, that would block the air to both the AAV and the idle valve. Maybe try just squeezing the line exiting the AAV. If the idle then drops, you know it is the AAV sticking. If not, check the hoses leading to the idle valve.

What happens if you screw the idle screw all the way in?

Does throttle plate close 100%?

Vacuum connections at back of intake OK? There are three back there - Transmission, locking system etc, MPS.

Tube on throttle body that leads to distributor via switchover valve properly connected?

[meltedpanda]

Hey Jim , yours is the 4493rd of the line, a distant relative
BTW Graham has arrived, he is the master D jet man, you ( we) are in good hands.
Where ya been Graham?
I agree double check that AAV

[Graham]

Originally Posted by meltedpanda

Hey Jim , yours is the 4493rd of the line, a distant relative
BTW Graham has arrived, he is the master D jet man, you ( we) are in good hands.
Where ya been Graham?
I agree double check that AAV

Firstly I am just a dumb owner who happens to have owned the D-jet for long enough (21 years) to experience most problems Have learned a bit along the way, but no mechanic!

I have done nothing at all to any of our cars this year except to take the snow tires off the smart car. I had the muffler fall off the 300D. Have a new one and it still awaits replacement! Working on house.

[blk914]

Ron and Graham, thanks for the replies and info. Adjusted throttle plate all the way closed sunday, this helped but still not right since there is no adjustment on idle screw, it's all the way down. Drove a couple of days, ran great till yesterday. Driving home from work it started a high surging idle, up and down. Squeezing hose to idle valve doesn't change anything, in fact you can cover the entire throttle body and it still runs. Getting air from somewhere, I suspect maybe a problem with the plenum gaskets. When i changed them over a year ago i couldn't get the ones that came off, they were not available. Replacements were shorter I think and I was doubtful they would even work. I do remember they were alot of trouble to put on. Guess I'll have to pull manifold to check, can't think of anything else it could be or any other way to check. Will keep ya'll posted.
thanks,
Jim

[Graham]

Originally Posted by blk914

Ron and Graham, thanks for the replies and info. Adjusted throttle plate all the way closed sunday, this helped but still not right since there is no adjustment on idle screw, it's all the way down. Drove a couple of days, ran great till yesterday. Driving home from work it started a high surging idle, up and down. Squeezing hose to idle valve doesn't change anything, in fact you can cover the entire throttle body and it still runs. Getting air from somewhere, I suspect maybe a problem with the plenum gaskets. When i changed them over a year ago i couldn't get the ones that came off, they were not available. Replacements were shorter I think and I was doubtful they would even work. I do remember they were alot of trouble to put on. Guess I'll have to pull manifold to check, can't think of anything else it could be or any other way to check. Will keep ya'll posted.
thanks,
Jim

I was recently looking at smoke pencils. They use them for detecting leaks in homes and duct systems. So called blower door test that pulls a vacuum on house.

It struck me that one of those might work well in finding leaks in a car intake. Even a incense stick or something like that might work. If you have access to a proper automotive smoke machine, so much the better.

It does sound like it must be the manifold donut seals or maybe one of the vac tubes off at the back of the manifold. I would look there first!

[meltedpanda]

sure does sound vacuum related , but dare I ask, how are the trigger points???

[RadioTek]

I hope that you've checked the "basics", your points dwell and timing.

I had weird problems until I swapped out my points with an electronic replacement.

Scott

[blk914]

Thanks guys for all the input. The more I think about it one of the plenum seals has got to be leaking, the replacements didn't even resemble the originals I took off. I remember having problems getting all to seat properly while assembling plenum to the manifold. I was told these were the only ones available (shorter with a lip and some type of re-enforcing ring around it). The originals were just plain rubber hose, the correct length. If this is the problem I might entertain getting the correct diameter hose and cutting my own ( should have done that from the beginning). I think timing is on, also running a Crane electronic ignition. One more thing, is this surging idle the ECU sensing the rpms going up with the throttle plate closed and it's cutting the fuel on and off? I read that on another site and was wondering if that was another quirk of the D-jet.

thanks again,
Jim

[Graham]

Originally Posted by blk914

One more thing, is this surging idle the ECU sensing the rpms going up with the throttle plate closed and it's cutting the fuel on and off? I read that on another site and was wondering if that was another quirk of the D-jet.

thanks again,
Jim

yes, the d-jet ecu does that - when rpm gets to about 1500rpm, it cuts of fuel injection. Then when rpm drops below some value that I can't recall, injection restarts - just a viscous cycle if there is a large air leak. Even a bad AAV will cause this.

Can you not buy the proper seals from an on-line MB dealer? Much better to buy the proper parts.

[blk914]

Update on plenum leak. Finally got to pull intake last week-end...just as I suspected, one of the plenum seals had failed. The seals I originally used were evidently aftermarket trash. Found out the hard way, try to use MB (when available) or quality replacement parts, it's worth it in the long run. Ordered new seals (new style are OK, was told plastic ring tightens as temps go up) got everything back together sunday, runs great! Thanks for all the great advise guys!

[meltedpanda]

great , these are always suspect, I suspect most of ours need some attention!