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  #1  
Old 05-27-2013, 12:52 PM
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Need ideas to diagnose lack of top end power 4.5

So I've had this problem for the past month since I got the car running, and it has not gotten any better no matter what I have done. The car runs great a low speed, I can spin the tires, it accelerates well up to ~40 mph. Likely it does not have -as much- power as it should, but it's not bad.
However, on the highway going ~60mph (~100km/h) anything past half throttle feels like I totally took my foot off the gas. If I downshift into second it feels like the engine died, and it will not have any power at all.
When it's 'on the edge', ie. just before I give it enough gas where it does this, it feels like it's misfiring.

Car:72 280SEL 4.5 with D-jetronic FI

What I've done:
-rebuilt engine
-new plugs and rotor, converted to Pertronix Ignitor, wires and cap look good, I did try another cap just to see
-Fuel pressure and volume checks out (~30psi, 3 liter/min).
-Air filter is clean
-Throttle position switch checked, cleaned, adjusted. Works fine.
-Checked MPS resistance, air and coolant temp resistance, all check out

Only thing I can think of is that the plug wires are shorting to ground under high loads? The plug wires look new, tho. I just put a cover around the coil wire so it does not directly touch anything that's grounded. The other wires do touch the valve covers in places. I know that's not great, but on most cars that how they runs and it's all fine. I do have some wire separators and I will play with them, see if I can route them so they do not touch each other or ground.

I have not tried adjusting the mixture knob on the back of the MPS.

Any thoughts on what this could be?
Thanks,

Csaba

PS.: My A/C leak turned out to be the compressor
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  #2  
Old 05-27-2013, 12:53 PM
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Sorry for the mistyping in the title...
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Old 05-27-2013, 01:54 PM
WANT '71 280SEL's Avatar
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It sounds exactly like the issue I had when my timing was way off. Also, check timing at 3k rpm. It should be about 27 or so with the advance working correctly. Also, on the adjustment, the distributor should be almost all the way to advanced.

Thanks,

David
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  #4  
Old 05-27-2013, 04:29 PM
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Oh, yes, I forgot to mention that. I did have that problem, but I got another distributor and now I get ~31 of total advance. I had to set the static timing to ~5 BTDC to get that (not 5 ATDC that the spec is).

Actually I am surprised that changing the distributor to one that advances so much more did not seem to make much difference.
Thanks,

Csaba
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  #5  
Old 05-27-2013, 05:21 PM
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I just pulled a plug after running full throttle at 40-50 mph, and they look very lean, FWIW.

Then I went for a drive and richened the MPS 1/3 turn at a time, 3 times total. The problem never went away, but it was somewhat lessened.
After the last adjustment it was rich enough that the plugs were black. Obviously too rich, so I went back 2/3 of a turn.

So whatever is happening, a rich mixture will lessen the effects.
bye,

Csaba

Last edited by vandor; 05-27-2013 at 06:39 PM.
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  #6  
Old 05-27-2013, 07:31 PM
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No answers, just a few questions:

-When you rebuilt the engine, did you put in new cams? New chain? Ensure the valve timing is correct? Asking because perhaps when your timing gets to 30+BTDC, perhaps it is too far advanced? And this would only happen at high rpm which is when you see the drop off in power.
-With same engine, I have never been able to get much beyond 28-29 BTDC with distributor at end of adjustment. My car runs best backed off slightly - about 27BTDC at 3000rpm. Are you at end of adjustment slot?
- Trigger points - are they original?
- Coil - did you install new coil with the Pertronix or is it just replacing points? Perhaps coil is breaking down?
- Plug wires - are they proper MB type wires with copper core? (The set of Bosch wires I have only cost about $70. (not exactly OE spec because they don't have resistor at plug end, but they work fine on my D-Jet. If your wires are shorting out, you should be able to see that in dark (dangerous with all those fuel hose connections!))

RE MPS - That is an awful lot to adjust the MPS. Should not need that.
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  #7  
Old 05-27-2013, 10:41 PM
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Hi Graham,

>-When you rebuilt the engine, did you put in new cams? New chain? Ensure the valve timing is correct?

It has used cams, a new chain and all new rails. I did make sure that the engines is at true TDC when the crank mark is pointing there, and the cam timing marks lined up.

>Asking because perhaps when your timing gets to 30+BTDC, perhaps it is too far advanced?

Well, it did this even with the previous distributor, which had a lot less advance. On my 'other' low compression engines it is normal to run 34-36 degrees of advance at higher rpm, so I just assumed the MB can use more. I can retard the timing to get a max of 27 and see what happens. It is pretty far at the end of the adjustment slot.

>- Trigger points - are they original?

I don't know. Visually they look good and I tested them as per the dwell procedure outlined in this forum and they passed. I do have 2 spare sets, so I could try one of those.

>- Coil - did you install new coil with the Pertronix or is it just replacing points? Perhaps coil is breaking down?

I used a new Bosch blue coil I had laying around. It would be easy to swap it out for another one and see what happens.

>- Plug wires - are they proper MB type wires with copper core? (The set of Bosch wires I have only cost about $70.

They are Bosch. They were replaced not long before I got the car. I will try the night-time test.

>RE MPS - That is an awful lot to adjust the MPS. Should not need that

I know, but I figured I'll make big changes to see if they make any difference.
Thanks,

Csaba
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  #8  
Old 05-28-2013, 10:26 AM
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Quote:
Originally Posted by vandor View Post
>- Trigger points - are they original?
I don't know. Visually they look good and I tested them as per the dwell procedure outlined in this forum and they passed. I do have 2 spare sets, so I could try one of those.
By dwell method, I presume you mean the angle that each set of points stays open or closed? Can't really see this being your problem, but you never know.

Quote:
Originally Posted by vandor View Post
>- Coil - did you install new coil with the Pertronix or is it just replacing points? Perhaps coil is breaking down?

I used a new Bosch blue coil I had laying around. It would be easy to swap it out for another one and see what happens.
I presume you wired Pertronix just as a points replacement if you are still using a Bosch Blue coil? That would mean that the MB switchgear is still in place.

Hard to see switchgear working at low speed yet being affected by engine load. Check if the red wire to the Pertronix is from 12V (ignition switch) side of 0.4ohm resistor. Needs a good voltage supply and some say it makes a difference. If you don't have a separate ground wire from distributor casing, adding one may also help.

Rest of stuff sounds as though it is good.

Have you checked vacuum at idle? It's interesting to advance timing with vac gauge attached until you get max (18" +/- ?), then back of just a little (should be around 14-16") then recheck advance at different speeds with timing light.

What would happen if car has a vacuum leak. Would that affect high speed performance? Too much air for amount of fuel being injected? No experience with this myself.
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85 300D,72 350SL, 98 E320, Outback 2.5

Last edited by Graham; 05-28-2013 at 11:08 AM.
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  #9  
Old 05-28-2013, 10:29 AM
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Sounds like your trigger points may be "Floating" at higher RPMs. There's some type of float by the sound be it trigger points, valve springs or something else. Are both valve springs installed on each valve and are they broken or weak at all?
There are some other potential causes:

> Wrong wires or wrong plugs. Are your wires resistance wires, or do they have resistance ends? If so please don't use resistance plugs. A set of 8 non-resistor plugs could solve the issue. I had a similar issue (it wasn't as bad as what you describe but I was lacking the push-you-in-the-seat kickdown power on resistor plugs).
Bosch 7505 Spark Plug | eBay

Sadly, most suppliers no longer carry the non-resistor plugs so you can also go with non-resistor wires to make up for that. The wires should be solid-core, the boots you can't get (to my knowledge) without resistors BUT you can get them in 1k ohm variance vs the default 5k variance, which can help recover that lost energy. You can call the classic center or Pelican for the best fit. I am unsure of the actual ohm value of the ones below but they're chrome, the stock have brass coverings which are 5k (I've seen the chrome on the 560SEL with 1k).
1972 Mercedes-Benz 280SE Sedan - Ignition - Page 1

The 560 ones are here and ARE a different part number:
http://www.pelicanparts.com/catalog/SuperCat/3482/MBZ_3482_ELIGNT_pg1.htm#item4
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  #10  
Old 05-28-2013, 11:18 AM
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Quote:
Originally Posted by Tomguy View Post
> Wrong wires or wrong plugs. Are your wires resistance wires, or do they have resistance ends? If so please don't use resistance plugs.
Tomguy
There is a never ending discussion on the net about resistor vs non resistor plugs, but no conclusive evidence either way.

I use Bosch/NGK resistor plugs with Bosch wires. The wires have less resistance that original (by about same amount as plug resistance). On my identical engine I have used that combination for 20 years and the 72 vintage car still performs per original specs.

Vandor likely has some other problem at moment.
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Old 05-28-2013, 03:25 PM
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If there was an ignition problem there would be backfiring as the unburnt fuel in the exhaust system ignited. Maybe your fuel pressure is dropping. Does the problem happen in every gear at a certain RPM?
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  #12  
Old 05-28-2013, 05:14 PM
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Quote:
Originally Posted by Graham View Post
I use Bosch/NGK resistor plugs with Bosch wires. The wires have less resistance that original (by about same amount as plug resistance). On my identical engine I have used that combination for 20 years and the 72 vintage car still performs per original specs.
I know it's an age-old debate! But you have the same overall system resistance. If Csaba has high-resistance wires/boots AND high-resistance plugs, he's not getting the spark potential he needs. The engine isn't missing per se, it just craps out because it's like a built-in rev limiter.

Speaking of rev limiter, do you maybe have a rev limiter rotor Csaba? Maybe it's got a weak spring and is grounding out too low.
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  #13  
Old 05-28-2013, 06:16 PM
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I know how it is with people throwing out possible answers over the internet without being there to look at the car, but I really think it's spark related. You're positive that your coil is up to snuff with the pertronix?

Best Regards,

David
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  #14  
Old 05-28-2013, 08:20 PM
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Quote:
However, on the highway going ~60mph (~100km/h) anything past half throttle feels like I totally took my foot off the gas. If I downshift into second it feels like the engine died, and it will not have any power at all.
When it's 'on the edge', ie. just before I give it enough gas where it does this, it feels like it's misfiring.
So, this problem is not related to rpm's, but speed?
Have you tried revving up in parking?

IIUC you have pertronix, but without Pert coil and still using the MB switchgear

.. Why dont you get the pert coil and bypass the switchgear? its easy.
This will make your setup simpler and if it doesn't cure the problem, at least you will have a reliable system.

Did you check your fuel pump still delivers at that speed?

Read the troubleshooting guide is very helpful:
http://members.rennlist.com/pbanders/DJetParts.htm#troubleshoot
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Last edited by Adv_rider; 05-28-2013 at 08:55 PM.
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  #15  
Old 05-28-2013, 08:53 PM
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Quote:
Originally Posted by Adv_rider View Post
So, this problem is not related to rpm's, but speed?
Have you tried revving up in parking?

IIUC you have pertronix, but without Pert coil and still using the MB switchgear

.. Why dont you get the pert coil and bypass the switchgear? its easy.
This will make your setup simpler and if it doesn't cure the problem, at least you will have a reliable system.

Did you check your fuel pump still delivers at that speed?
I did ask before about the Pert. If the switchgear has been bypassed and a blue Bosch coil used, there would likely be problems. Your suggestion is a good one.

Re the pump. The OE pump will only do 2L/min vs 30psig head (measured on return line from FPR/damper) If the pump does 3L/min as mentione earlier, and measured at same place, is it a proper D-jet pump? Hopefully the injectors are clean and not the problem.
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