Rebuilt 4.2 L

I just had a 1988 420 SEL 4.2 Litre motor completely rebuilt at a reputable shop. Everything seems fine, leak down test, wet and dry compression test -all checks out o.k. When at idle, the motor visibly shakes and seems to be missing. The mechanic is even at odds here, and might send it outside to determine the problem. Of course everything was installed correctly, timing set, plugs gapped, etc. Could this be a vacuum leak, fuel/air mixture issue or some relay at fault here? Any suggestions greatly appreciated.
Tony.
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Couple of things...
1. Spark plug wires... measure the resistance... they should all be under 1300 ohms.
2. Idle speed air valve should be cleaned with carb cleaner until it is clean.
3. Fuel pressure regulator (probably original) should be changed out.
4. To determine vacuum leak at all gaskets and hoses, use carb cleaner and spray heavily... if engine stumbles or changed, you got a leak.
5. Fuel distributor air/fuel mixture should be readjustd... with rebuilt engine, it needs less gas to run.
6. Engine mounts? Should be new.
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Question: Does it shake on cold start and at hot idle, or only when hot?
Does the motor have good power when you drive the car?
Possibilities (no insults intended):
A bad spark plug. Or a bad ignition wire; check each with an ohm meter. Two spark plug wires are switched, yes it could happen.
A bad fuel injector or incorrect fuel system pressure.
A vacuum leak as you mentioned, probably at the manifold to cylinder head joint. That can be found by CAREFULLY applying propane to all suspect sites (Standing well back so if it flares you don't get burned). The propane will get pulled into the motor and make it run well.
Has the valve timing been checked? One bank may be off by a tooth.
If none of these ideas resonate a compression test may be the next task to diagnose the problem.
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It's a long shot but I saw a case like yours one time that the mechanic that had done the engine couldn't get it to idle smooth. Turned out to be a new rubber part. On each side of the engine between the valve cover and the injectors is a rubber hose piece that has 4 niples that hook to the 4 injector shields. One of the niples was pluged up. The engine gets idle air through these niples hince one cylender got no idle air and would miss. If you take off the hoses and look in each niple you can see if it is blocked. This is a very hard problem to diagnose, but an easy one to fix. Paul
<DIV><FONT face=Arial size=2>I just had a 1988 420 SEL &nbsp;4.2 Litre motor completely rebuilt at a reputable shop. Everything seems fine, leak down test, wet and dry compression test -all checks out o.k. When at idle, the motor visibly shakes and seems to be missing. The mechanic is even at odds here, and might send it outside to determine the problem. Of course everything was installed correctly, timing set, plugs gapped, etc. Could this be a vacuum leak, fuel/air mixture issue or some relay at fault here? Any suggestions greatly appreciated.</FONT></DIV> <DIV><FONT face=Arial size=2>&nbsp;&nbsp;&nbsp; Tony.</FONT></DIV></BLOCKQUOTE></BODY></HTML>
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I did if fact change out the 2 pieces yesterday. In both pieces there were cracks at where the nipples left the main crossbars. It helped a little, slightly noticeable. Not there yet, my mechanic sent it out today to another shop, he is stumped as well. New plugs, wires, timing chain, (timing set of course). It seems that was the only noticeable vacuum leak. Now the oxygen sensor will be replaced, it connects at the exhaust and wires to an inboard computer under the passenger side inside cabin in the firewall. We're still working on it.....
<DIV><FONT face=Arial size=2>I just had a 1988 420 SEL &nbsp;4.2 Litre motor completely rebuilt at a reputable shop. Everything seems fine, leak down test, wet and dry compression test -all checks out o.k. When at idle, the motor visibly shakes and seems to be missing. The mechanic is even at odds here, and might send it outside to determine the problem. Of course everything was installed correctly, timing set, plugs gapped, etc. Could this be a vacuum leak, fuel/air mixture issue or some relay at fault here? Any suggestions greatly appreciated.</FONT></DIV> <DIV><FONT face=Arial size=2>&nbsp;&nbsp;&nbsp; Tony.</FONT></DIV></BLOCKQUOTE></BLOCKQUOTE></BODY></HTML>
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I now have my car back, returned from a different MB repair shop, that has been in business for 30 years. I still have a problem with the idle, and if the engine is cold, I have to hold down the gas peddle for it to start. Any suggestions? I have had the air/fuel mixture readjusted by them but was told that sometimes the older motors shake - it didn't before the total rebuild.
<DIV><FONT face=Arial size=2>I just had a 1988 420 SEL &nbsp;4.2 Litre motor completely rebuilt at a reputable shop. Everything seems fine, leak down test, wet and dry compression test -all checks out o.k. When at idle, the motor visibly shakes and seems to be missing. The mechanic is even at odds here, and might send it outside to determine the problem. Of course everything was installed correctly, timing set, plugs gapped, etc. Could this be a vacuum leak, fuel/air mixture issue or some relay at fault here? Any suggestions greatly appreciated.</FONT></DIV> <DIV><FONT face=Arial size=2>&nbsp;&nbsp;&nbsp; Tony.</FONT></DIV></BLOCKQUOTE></BLOCKQUOTE></BLOCKQUOTE></BODY></HTML>
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You need to change the engine temperature sensor for the fuel injection system... there is two on your car... This will tell the computer whether you need more fuel for cold engine.
I am assuming your hot engine idles just fine... normal speed. If not, then air valve and idle speed controller is involved. Idle at hot engine is around 600 RPM... cold is 800 RPM.
Also check for vacuum leak. Make sure all hoses are snug.
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Did a valve job on a 91 420 SL a while back..everything worked fine except it ran rough. turned out one of the injectors (which laid on the garage floor for the duration--Well DUH) had picked up some dirt, dog hair, or something (discovered and cleaned by a indy shop in Alameda)
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