A buddy of mine is trying to get rid of his 420SEL (gas engine), which he says has terrible engine performance.... He says the pedal has to go to the floor before anything picks up, but once it's at full speed then it stays that way... He's been told it's a 'vacuum' issue, which I think is BS because I don't believe the vacuum has anything to do with the engine or transmission functioning other than performance/optimization/shifting/etc. He took it to a shop that supposedly got it to run on all 8 cylinders (how it was not running on all 8 I dunno), and came home with it in WORSE condition.

I'm looking at the car tomorrow to take it off his hands and I'm wondering if there's anything obvious to look at that could significantly affect performance/power? If it were a diesel, I'd consider something with the throttle (my 300TD had a broken throttle bushing that affected the power), or compression or maybe new injector tips). But I've yet to see the vehicle and aren't familiar with gas engine issues as much.

Any ideas?

i think a good close visual inspection is needed and after that idle or drive the car on a short trip until its a operating temperature and do a compression test.
When was the ignition parts changed?

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

Would a compression test rule out the engine itself (ie internal damage/wear)? I just ordered a diesel compression test kit from mercedes source and am not sure if maybe I have the right connector to use that on the gas engine? or maybe the gauge is all wrong and i'd need a whole new gauge for gas engines.

What items with the ignition are you referring to? The owner may better know about specific parts, etc. Also, it would help for me to know what to take a look at.

The owner said it was working fine but then performance changed all of the sudden in one day... I forget what he said, but it seemed like nothing major happened.

There is not much inside the engine really so a compression test can be valuable. But sticking valves in the heads can be a bad thing. But I have never seen them stick enough to drown an engine out of power.

Few things though. Fuel delivery issue, air intake issue or timing issue. If the timing chain was replaced and its off, then there could be an issue. But I dont think that is too likely unless there are other issues. But its a thought.

There is a thing called an OVP relay. Its usually a 10 amp fuse under a cover along the back firewall. Driver side. Mines has a U.S. style fuse, not a euro type. If this goes I guess there can be some weird issues.

The fuel mixture could be way off. That can be adjusted, many threads on how to do that. But I would start with a new O2 sensor to be sure its not bad.

Really there are a lot of things that come to mind. As suggested it needs a good visual once over to look for anything obvious. Check for vacuum leaks, etc. Then move onto putting in new plugs just to be safe. After all they are cheap and when you remove the old, not the locations. The condition of the plugs can indicate certain issues. Wires, if original, are a good idea. The list can get pretty long/expensive. So start with the basics, move onto plugs, then sensors, blah blah....

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

How might vacuum leaks affect things? I was under the impression that other than shifting, the vacuum is more for things like the brake booster, power locks, ignition cutoff, venting, etc. Does it tie into the air intake at all (like how the diesels have an ALDA that connect to the turbo)?

I have a US General Compression Tester, but it has diesel adapters, I'm wondering if it would work on a gas engine too? I'm not sure if any of the adapters would fit in the spark plug hole (maybe the one on the lower left?): (the kit model is 93644)

If I rule out engine internals from a compression test, then would that indicate that any problem would be external to the engine, ie much more likely/cheaper fixed? I just want an easy way to see if the car is worth tinkering with (ie manageable by myself, within a budget and not requiring major work) or better to part out.

Its a gasser, vacuum is everything. If there is a leak, sure it can hurt performance. The vacuum systems run right off of the intake manifold. If there is a leak then it usually has a really bad idle or wont get any power at higher RPM. From what you describe it sounds like a few problems.

And FYI, the locks have their own pump in the trunk to operate the vacuum lock pods. Vacuum is not used for ignition cut-off, ignition switch kills spark instead. But it does tie to the brake booster, HVAC, Evap system (fuel tank fume recovery) and a bunch of other little things like EGR valve.

Im tired, so I am forgetting things probably but I hope it helps a little.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

I would guess yes, it would be external items. For a 420 to have such bad power it would have to have some serious blow-by!

As for the kit you have, I cant help. I always have compression tests done by a friend. But I think his just has a rubber end he holds to the plug hole by hand and another turns the engine over. Also I am not sure if it will work for a gasser, but I dont see why not. It just reading how well the air compresses within the chamber. And a diesel is much higher compression. I would think the kit would have a large range.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Rule out clogged cats, assuming it had them.

Improperly adjusted throttel linkage can cause rotten trans shifting problems, which results in doglike performance. The control pressure cable and vacuum modulator should be checked.

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Prost!

whenever you have performance issues do the simple stuff first. A Compression test will eliminate areas to look at and tell you the mechincal health of the motor. The diesel compression tester wont work. ignition parts to look at include plugs, wires, distributor. Make sure those are good. I would then make sure fuel pressure is good.

Once the basics are established; once you determine where your engine compression, fuel pressure and no vacuum leaks and at and its in spec then you can start looking at other areas that can cause performance issues then you can look at engine timing, fuel injection, evap system.

another thing to consider even Though i havent road tested the vehicle is it could be the torque converter. If it takes forever to acceralate and you have determined nothing is wrong is with the engine it could be a bad torque converter. as mentioned below check to make sure the vacuum modular on the tranny is working fine. to determine if its something with the transmission disconnect the Vacuum line to the transmission. this will result in harder shifts but tell you if the tranny is shifting fine and the torque converter is working

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)