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  #1  
Old 08-11-2005, 09:42 PM
bhatt's Avatar
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Join Date: Apr 2000
Location: Oakville, Ontario, Canada
Posts: 421
Exclamation M104 hesitation - 300SL

Hello all,

I am having a hesitation problem with my 1990 300SL (CIS fuel injection). Here are the details.. what should I look at next???

Problem:
-Hesitation when accelerating from idle.
-No problems with starting engine, idling, or once car has started moving. Accelerating from 5mph-60mph is perfectly fine!
-Anything other than light pedal pressure causes it to bog down for a second before it accelerates. Hard acceleration causes it to stall!

Conditions:
-Happens consistently all the time (cold weather, just after starting, when fully warmed up, etc, etc.)

What I've done:
-Replaced fuel filter
-Had fuel injectors flushed/cleaned
-Replaced air filter
-Replaced cap, rotor, spark plugs (but not wires - they are in excellent condition)
-Sprayed spark plug wires on both ends with mist of water while running
-Looked at wires in dark (no arcing seen)
-Pulled one spark plug wire and tested - 1230 ohms (spec says max 1200 but I'm sure that's not the problem since it's so close)
-Visually checked for disconnected vacuum hoses (none found)
-Cleaned throttle plate with carb cleaner
-Cleaned air temperature sensor with electronic parts cleaner
-Ran car at 2000rpm and pressed on throttle microswitch (the deceleration fuel cutoff switch). Car slowly dropped a few hundred RPM in and went back up when I released the switch.

WHAT'S LEFT? It's driving me crazy! Not sure where to go from here?!?!?!

Any help would be appreciated! This is such a simple fuel injection system but it's driving me nuts.. car is virtually undriveable...

Thanks
Neal

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'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)
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  #2  
Old 08-11-2005, 10:01 PM
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Join Date: Dec 2003
Location: Fribourg, Switzerland
Posts: 277
So that's one of these few m104 with CIS? That might be the potentiometer of the air flow meter. It tells the electronics the amount of air entering the enginge so it knows how much fuel to inject. Typically the potentiometer wears out first in the range above idle. It is not very difficult to replace and costs around 45$ from europartsdirect.com (not available from dealership or fastlane).
Ignition or fuel problems would be even be more evident at higher revs, and idling is fine too you say.
Good luck, Bruno
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1992 300TE 160 kmiles
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  #3  
Old 08-12-2005, 06:48 AM
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Join Date: Aug 2002
Location: Southeastern PA
Posts: 3,077
Neal, check the O2 sensor. I had a similar hesitation problem with our 1991 300CE with the 104 engine and replacing the O2 sensor cured the problem.
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  #4  
Old 08-12-2005, 10:35 AM
Lenny
 
Join Date: Jun 2005
Location: Richmond, Va
Posts: 73
104 hesitation

My experience is with a 103 CIS. However, I went down the same path that you did and found nothing wrong. I solved the problem by adjusting the mixture screw in the top of the fuel distributer (trial and error). As I recal, iIhad to adjust for a leaner mixture v. what the exhaust gas analyzer would dictate.
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  #5  
Old 08-12-2005, 02:57 PM
MrCjames's Avatar
California Dreaming
 
Join Date: Jun 2003
Location: SF Bay Area
Posts: 599
Sounds like one of the two fuel pumps may have failed......................OR you may have a bad ignition coil. IMHO
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  #6  
Old 09-13-2005, 09:48 AM
bhatt's Avatar
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Join Date: Apr 2000
Location: Oakville, Ontario, Canada
Posts: 421
Update

Hi guys..

An update. I did a bunch of stuff as follows, then gave up and took it to my tech to fix it. Here is what I did:

-Cleaned the air flow meter plate
-Replaced the Air Flow Sensor Potentiometer
-Tried to check out the O2 sensor but couldn't reach it.. tech said that wouldn't cause the problem because there is a "lag" (1-2 second delay) before the O2 sensor can affect anything. My problem happens immediately when you hit the gas.

He ended up replacing the EHA and said the car was much better (I haven't picked it up yet). He made me drop off the old air flow pot, as he said the car wasn't perfect with the replacement one.

He could adjust the new MAF pot to idle properly, or have perfect power at full throttle, but not both. When the idle was good, you got a lag when pushing the throttle down 3/4 of the way or more. If you adjusted the pot to make that perfect, the car wouldn't idle properly.

I dropped off the original MAF pot and he called to say it was much better. He said it isn't completely perfect (he's a perfectionist) so he wanted to keep the car overnight and adjust it cold.

Just thought I'd pass that info along for the benefit of the forum. My MAF pot looked worn out (I will try to post a pic if I can find it) but that was NOT the problem! Replacing it actually made things worse.

Apparently replacing the EHA fixed everything. Will drive it this evening and post an update when I get a chance.. His words on the phone described the improvement as "night and day" from when I dropped it off. I'm excited to go and pick it up..

PS. I will post shortly in the for sale section, but if anyone needs a MAF pot, I have one that was only used for a day. If anyone is interested, let me know before I eBay it.

Neal
__________________
'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)
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  #7  
Old 09-14-2005, 12:52 PM
bhatt's Avatar
Registered User
 
Join Date: Apr 2000
Location: Oakville, Ontario, Canada
Posts: 421
Another update...

The car is now perfect. Tech still says it could use a bit of improvement, but it is 100% better than when I dropped it off.

Car won't stall at all (AC on or off!) and idles incredibly smooth!

Definately check your EHA before replacing a MAF potientiometer. Apparently my EHA didn't test bad, but it was. I'm told that the EHA will pass the electrical test in the manual, but can still be bad.

If you are having a similar problem and find this thread, email me if you need help on what I did, tips on what to look for, or any other info.

Neal

__________________
'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)
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