Help! 560SL loses power above 4000 RPM's
 

I bought my 88 560SL last October and have replaced most of the usual suspects for a car with 120K miles on it.

* both fuel pumps and filter
* Spark plugs, wires, dist. cap, rotor, coil, air filter
* timing chain, rail's and tensioner
* O2 sensor
* Sub frame/motor mounts/flex disk
* Fuel injector seals

The car starts every time on the first crank and idles fairly smooth and steady at about 650 RPM's in gear, and 750 RPM's in neutral.

The engine runs smooth and strong up to about 4000 RPM's where it just seems to run out of power. It will pull to 6000 RPM red-line but with reduced power. It feels like the fuel flow is reduced or that the timing has been retarded and at the same time the engine takes on a different sound. Not any strange noise, but a change in frequency. This problem only happens between 4000 and 6000 RPM.

Could this be a blocked Cat??? Bad injector??? I am running out of ideas.

Help!! any input is greatly appreciated....

Anyone out there have any ideas??????????????????

I'd lean towards the cat. Easy to test, I'd just loosen up the front exhast pipes at the exhaust manifolds and see if the problem is still there or not on a roadtest.
Gilly

I am kind of leaning toward the cat too. I may try to unbolt the exhaust from the manafold and see if there is a difference. The only problem is that with open exhaust I'll be a cop magnet. Is there a way to check the Cat back pressure th see if its blocked???? Thanks for your help

There is a way of doing it Jerry, I believe a vacuum gauge can be installed, if I'm correct, the vacuum should be unusually low. I usually just do the simple test I've outlined, it gives alot more positive results. Just loosen all the bolts attaching the pipe to the manifold, no need to remove anything completely. Never had a problem just doing this as a test. In the unlikely event you would be stopped, I'm sure most officers will understand. Don't do it at 3 am obviously, and I would assume you would know within a mile or two if the problem is gone, then cork it back up until you can fix it. You'll feel alot better doing it this way rather than hoping that you are analyzing the test results properly, this will be a very expensive repair.
Gilly

Dan, thanks for the insight. It will be a few weeks before I will get a chance to crawl under the car and uncork the exhaust but I will give it a shot and post the results. You indicated that if the CAT is bad it will be an expensive repair... How expensive are we talking???? Is there an after market replacement???

Part of the reason that I won't be able to look at the CAT problem is that my priority just got changed on me. Last weekend out of nowhere my air conditioning just quit and I can deal with reduced porew above 4000RPM more than no air conditioning in commute traffic. The compresor doesn't kick in at all, but I can run a "hot" wire to it and the clutch kicks and it begins to turn so I am thinking a relay or switch somewhere. I have checked all fueses and they look good. What is strange is that o few times before it quit for good, it would stop cooling and if I turned the AC off and back on it would start cooling again. I don't know much about AC so unless I find a bad switch or sensor I will have to take it to an expert... Any ideas??? Thanks again for the help

Sounds like a low freon issue especially considering it was going bad before the compressor finally quit. I would consider taking it to an a/c shop and having them put the gauges on it and see if they can tell what is going on.

I'm with Jim, this is the usual scenario as you have outlined. I'd try jumpering the low pressure switch to see if the switch is closed 9by closing the wiring connection) if the compressor runs. Don't leave it jumped, "this is only a test".
On the cat, this could literally be a few thousand dollar proposition to do it with MB parts, i'm not aware of what's available in the aftermarket to replace it.
Gilly

Thanks Guy's: I Did as you suggested and jumpered the low pressure sensor on the AC and the compressor kicked in. It looks like I just need a re-charge and have talked to a couple of shops near where I live and have gotten quotes for R12 from $59 lb. to $159 lb. to recharge and one guy told me that I likely have a leak and should have it converted to R134 and the other says that the old systems just run low after several years. The bottom line it that I plan to take it in next week and if all it needs is a re-charge I will only be out $130-$160 unless there is a leak. What is your experience, are these systems prone to have leakes or is it likely that a re-charge will do it????? Thanks again for the help...PS: I'm still a few weeks out from having time to unhook the CAT.. I'll keep you posted....

AC diagnosis
 

Have the AC tech put some flourescent dye in with the charge up.
If you have any leaks, they will become obvious.

Did that on my 85 SEL, replaced the faulty hose, and have had nice cool air for two summers now.

LOL Michael

I had a similar loss of power problem at 3500RPM om my C180 (European MB model). It would idle fine but as soon as the pressure built up in the exhaust system the blocked CAT would refuse any increase in Revs. A replaced CAT solved the problem albeit at some expense and hassle during fitting..I do not have a pit im my garage and so have to lie prostate under the car.
Hope this helps.

Update on high RPM loss of power... I ran the car with the Cat un-bolted with no noticeable change, but when I pulled the spark plugs out to inspect and re-gap I noticed that 3 plugs looked a little "wet" from gas. The color looks good ( light tan ) so it doesn't seem to be running rich. My thought is that I may may have some fuel injectors that leak. It has also begun to idle a bit rough in gear and when in park the RPM seem to fluctuate slightly. ( maybe +/- 50 RPM's) Does it look like I am on the right track??? Thanks for any input...

Update on AC problem: I had the system recharged and the tech found a big hole in the hose that runs from the receiver/drier through the firewall and under the dash. They charged me $150 to charge (1lb of R12) then purge the system and told me they would have to order the hose from the dealer then flush the system and recharge it. They want $350 to flush and fully charge (2 1/2 lb. R12) the system plus whatever the hose and labor to install it will be. I thought about buying the hose and installing it myself, but if it leak's at any connection when they recharge, they will purge it again and charge me another $150 and my AC still might not work. It looks like when working with R12 there isn't much you can do yourself. Maybe its time to consider converting to R134. Any ideas on how much that woud cost??? Thanks for any input...

It will cost you good A/C.

Steve: are you saying that I would be better off spending the same $$ to fix my R12 system than to convert to R134???