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  #1  
Old 09-09-2019, 12:05 AM
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Need help! 280SE 4.5 stumbles and won't go over 20mph

Hi guys,


Have put 6000 relatively trouble free miles on it.


This week, it had a little hiccup going up a hill which quickly evened out, and a bit more during random acceleration.


I changed my oil Friday night and suddenly would run like **** under any load, idle is still fine, but any throttle and it starts to shake and run bad. It won't get over 20mph.



1) checked the plugs and coils which were replaced this year, everything seemed ok


2) checked fuel pressure, good at 32psi


3) oil pressure is good, cooling system is working


4) timing is still where I had it, 10btdc


5) oil level is correct


6) cap and rotors seem fine, they are relatively new as I installed a pertronix back in November



The only work I have done recently is the oil change and replacing the timing chain tensioner with a genuine mercedes one, as I had some chain noise at idle before the pressure would come up. Hasn't been back since I put the new tensioner on.

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  #2  
Old 09-09-2019, 12:38 AM
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Have you checked the fuel pressure at higher throttle positions and maybe under load. Maybe your fuel strainer or filter I partially blocked, and it can supply the amount of fuel required to activate your fuel erasure regulator, but when it starts supplying fuel to the injectors at higher rates, the fuel pressure drops.


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Past mb: '73 450sl, '81 280slc stick, '71 250, '72 250c, '70 250c, '79 280sl, '73 450sl, parted: '75 240d stick, '69 280s, '73 450slc, '72 450sl,
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  #3  
Old 09-09-2019, 12:49 AM
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Quote:
Originally Posted by fonzi View Post
Have you checked the fuel pressure at higher throttle positions and maybe under load. Maybe your fuel strainer or filter I partially blocked, and it can supply the amount of fuel required to activate your fuel erasure regulator, but when it starts supplying fuel to the injectors at higher rates, the fuel pressure drops.


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I checked higher throttle but not under actual load. But the bad running is very evident at higher throttle so I figure it would show then.
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  #4  
Old 09-09-2019, 02:05 AM
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Will be pulling the distributor to check trigger pts tomorrow.
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Old 09-09-2019, 08:16 AM
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Did you knock off a vacuum hose or other line. During oil change? If it was running well prior and now not check your steps TP are always a suspect. Lol. Especiallt the wite connecting harness
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Old 09-09-2019, 09:46 AM
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If you are at 10degBTDC, is that with the vacuum connected? Seems high. I think spec is from 5degATDC to TDC depending on model. I think I have my car at about 5deg BTDC because it runs better that way. These are all with vacuum to the distributor connected. As you increase rpms to 3000rpm, you should see the timing go to something like 27deg BTDC. This will show that the centrifugal advance is working.

So maybe check vacuum line is connected and that nothing is preventing centrifugal advance from working.

If engine idles well (throttle plate closed) but doesn't run well once throttle plate opens, it could be a mixture problem. Idle uses a different mixture control circuit in ECU. One throttle opens, the MPS and temperature sensors come into play. At least check that the connectors to all injectors, MPS, TPS, trigger points and temp sensors are properly seated.

The MPS depends on engine vacuum. You should see about 16"Hg of vac at idle and this should gradually reduce as you increase rpms. If there is a vacuum leak, mixture will be wrong. Even a stuck open or closed AAV will cause a problem. Open would give you high rpms at idle though.
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Old 09-09-2019, 10:29 AM
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All the vacuum lines are connected. Vacuum is good at like 16-18 range. It drops a bit under throttle then goes back up right away.
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Old 09-09-2019, 10:55 AM
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I have had this issue with my 4.5 in the past. Not right after an oil change, but I've had it.

Pull your D-Jet trigger points. I bet they're fouled with oil and you're only hitting 4 or 6 cylinders under load as a result.
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Old 09-09-2019, 11:03 AM
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Originally Posted by Tomguy View Post
I have had this issue with my 4.5 in the past. Not right after an oil change, but I've had it.

Pull your D-Jet trigger points. I bet they're fouled with oil and you're only hitting 4 or 6 cylinders under load as a result.
Thanks Tom, I read this suggestion from you in a prior post so it's on the agenda today.


Is there anything special to pulling the distributor beside re-timing it after?


BTW guys to clarify, I did have hints of the issue the week before the oil change, it just became constant after the oil change, so maybe a coincidence or just exacerbated it somehow..
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Old 09-09-2019, 01:46 PM
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I had 350SL in about two years ago. It ran OK but had no power under load but it would go faster than 20 MPH before it ran out of power.

Turned out to be a bad throttle switch. I didn't suspect it because the switch was new and also faulty. Changed the switch and it would pin you back into the seat.
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Old 09-09-2019, 02:14 PM
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That's a thought, I did check it over a weeks ago and verified it was working with a multimeter, but I can check again.
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Old 09-09-2019, 10:17 PM
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Hey Graham, hope you are well.

Bezant - Graham is your man with all things trigger points - you are in good hands if that is the issue
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Old 09-09-2019, 11:29 PM
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Hi guys,

more digging tonight but not resolved. I pulled the distributor off and the injector points seemed fine.





I gave them a quick spray with electronic cleaner but it didn't change anything.

You can see a quick rev here and how it dies out as revs increase.
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Last edited by Bezant; 09-09-2019 at 11:43 PM.
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Old 09-10-2019, 09:23 AM
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Originally Posted by meltedpanda View Post
Hey Graham, hope you are well.

Bezant - Graham is your man with all things trigger points - you are in good hands if that is the issue
Hi Ron. Still doing OK. my old Djet doesn't get used much, but runs really well.

TP's look OK - the rubbing blocks don't look very worn. It's when they wear that pulses to ECU are affected.

Pertronix is good, but did it just replace points, or switchgear as well? Type of plugs, HT wires? I would check all wiring/harness connections.

Timing - as mentioned, may be overly advanced.

Could be several other things.
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Old 09-10-2019, 10:11 AM
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To elaborate on Graham’s timing comment.

Timing for a US 4.5 should be 5 degrees AFTER top dead center with vacuum at 750 rpm. When AC is turned on, vacuum should be shut off, and timing should advance to 5 degrees Before TDC.

So regardless of whether your timing is with or without vacuum, your too advanced. You will get great torque off the line, but it won’t run right at higher rpm. If you are 10 BTDC with vacuum, you are 15 degrees too advanced. If 10 BTDC without vacuum, then it’s probably not THAT big of a deal.

Can you answer whether your timing is with or without vacuum?




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