72 280SE 4.5 hesitation

[Scotto52]

So I had posted a little while ago about a hesitation problem and was given some great advice on where to start looking. But I still have this bit of a hesitation when giving it some gas. To the point of just trying to get across an intersection it just stumbles and stalls out. Starts right back up no problem. So here's where I'm at...

I have no history on this vehicle. Odometer is stuck at 75K.
So far I have checked to make sure the chains are not stretched,
adjusted the valves,
removed the distributor and installed a Petronix II ignition kit,new plugs and cap, replaced the spark plug wires with non-solid core type (as per Petronix). Inspected the trigger points for oil and such. No play in the distributor shaft. Vacuum advance moves and holds vacuum. I set the timing by using a vacuum gauge to find the most vacuum and set it back a bit. I think i was close to about 20hg and set it to about 17hg with the vacuum advanced port plugged when warm. The idle is about 700-800 rpm's, I've been kinda playing with the ignition timing. I also think my timing was about 27BTC (its tough to see, time for bifocals).

I have replaced the fuel filter, the other one was extremely dirty. I even took the back of the pump off to make sure it wasn't tarnished, and it wasn't. Fuel pressure going up to the fuel rail was 30+psi. But when I hooked it up today to drive, I would have fuel pressure but much lower. Maybe around 20psi on acceleration. (again bifocals, kinda hard to see the gauge at the front of the hood) and the car wasn't hesitating going up and down the alley !?! I put it back together and it actually seems like it's running better. I hit 2nd gear no problem. Possibly the placebo effect also.

Other factors they may or may not matter...
-checked the TPS with KOEO and listened for the 20 "clicks", all good there
-orginally I didn't have any vacuum at the distributor at idle. That electric solenoid that cuts vacuum to the distributor when the AC is on, wasn't working. So I bypassed that. The AC doesn't work and I never put the drive belt on the compressor.
-I want to change the strainer in the fuel tank, just waiting to burn some fuel off. And it's new fuel.
-there's no high idle when cold. It has also been about 90° the last few days. I read of this valve sticking. But I don't feel that's the problem. If I set the ignition timing when its hot, I would think it doesn't matter if the valve was stuck or not.
-I sprayed a slight amount of carburetor cleaner in the throttle body to see if the idle went up, it didn't. I would think that means I don't have a vacuum leak. Ruling out leaky intake gaskets, map sensor, cold idle valve, etc. (vacuum door locks still work even after a few days of sitting)
-I changed the hose going to the MAP sensor (or whatever the proper name is). I've been reading that this is a very sensitive sensor. And costly. I need to ohm out the two coils to see if I have continuity.

So, few questions are...

-Should the fuel pressure be a solid 30+psi when driving? Or will it fluctuate and drop down to about 20psi?
-Is there any sure way to check the MAP sensor? I've read the best thing to do is try a know good sensor. I guess checking the coils for an open would be a sure failed sensor.

Pretty sure I have another question rolling around in my head somewhere. Thank you in advance for any input, advice or thought. Stay safe!

[Graham]

Fuel pressure must not drop. It needs to stay at 30psig. Do you have the original djet pump? If not you should! Measure flow leaving fpr/ damper. Should be 1L per 30 sec.if your pressure drops, could be a pump capacity problem.

Testing MPS is covered in FSM. Needs to be checked for resistance and vacuum.

Most of us used Pertronix 1 with copper wires. Are you using existing coil, switchgear and resistors? If problem persists after you get fuel pressure sorted, maybe spark is weak?

[Scotto52]

I’m not sure what the original pump actually is. I watched a video by Kent from MS and it was exactly the same pump, so I want to say yes. I’m using the Petronix II and reached out to them on the best way to wire it up. So again I’ll say I’m using the switch gear. ( used the green wire off the distributor and the other to the side of the blue resistor.) I also had the same exact problem before with the points. Being that fuel pressure needs to be constant 30+psi, (I mean, it makes sense, but I just wasn’t sure), i’m going to say the fuel pump and my strainer in the tank is my next move.

Is there a replacement pump that everyone uses around here that works good?

Thanks!

[Graham]

There is a recent post about a pump for a 300sel, I think. Towards the end I posted about a Nissan pump that is available and is identical to the original Bosch pump.
When you get the fuel sorted, you might want to post more about how the Pert is wired.

[aluminum]

Clean all the Grease / Oil film off from the Drivers side inner Fender panel lower area's front . Spark leaking to ground can be a issue that can cause issues . This area you can simply pressure wash and let it dry .
Yes the fuel pressure must be constant .
Is hard to check the Trigger Points they may well be a issue as well . No oil on them is good of course but what happens is the Fiber wears down where the cam rotates and they don't send the signal for the injectors to work right or get Fuel

[Tomguy]

Also, if you bypassed the vacuum solenoid so you always have vac at the distributor, don't do that. Set your timing without the vacuum line connected, and just plug the source (the port on the throttle body). The vacuum solenoid is supposed to shut that vacuum off when you're not idling, I believe (I forget the scenarios where there's vacuum but it's pretty much just idle with no A/C and under 100C, it's a vac retard).

You stated timing is 27 BTDC - I assume this is at 2500-3k RPMs, not idle. If that's idle, it's way too advanced, and could be part of the issue.

The other thing that could be happening is your idle mixture is too lean, and when you add throttle, D-Jet doesn't get the fuel flowing quite fast enough, hence the stumble and die. There's a screw on the side of the ECU. It should be set "In the middle" - turn it until your idle starts to slow, then turn the other way and count the clicks until it starts to slow again. Divide by two, set it there. Unless you have a CO meter to "Do it right".

There are two adjustable items off your fuel rail. One is a pressure regulator, another is a "Dampener" - perhaps your dampener is adjusted too far in? It's supposed to keep you from being able to hear injector pulses, but if a sudden fuel surge to the injectors causes a drop to 20PSI, it's possible this is too aggressive.

[Graham]

Sorry Tomguy, a few comments

Originally Posted by Tomguy

Also, if you bypassed the vacuum solenoid so you always have vac at the distributor, don't do that. Set your timing without the vacuum line connected, and just plug the source (the port on the throttle body). The vacuum solenoid is supposed to shut that vacuum off when you're not idling, I believe (I forget the scenarios where there's vacuum but it's pretty much just idle with no A/C and under 100C, it's a vac retard).

Idle timing is supposed to be set with vacuum connected. Manual may say at 5ATDC, but we find that TDC->5BTDC works best. Vacuum at idle retards timing by about 10-12deg, I believe. So, if timing is set without vacuum, maybe it could be set at 8-10BTDC for a start? Using vacuum as Scotto did is a good way to set initial timing. I do that, then check it with timing light. Confirms that vacuum is good . If centrifugal advance doesn't get you to 27BTDC, but close, the advance idle timing a little. If you are nowhere near 27BTDC, the centrifugal weights may be sticking. Rebuild article is available.

You stated timing is 27 BTDC - I assume this is at 2500-3k RPMs, not idle. If that's idle, it's way too advanced, and could be part of the issue.

Correct, but I was not sure at which point Scotto thought he may be at 27BTDC. That is the target at 3000rpm.

The other thing that could be happening is your idle mixture is too lean, and when you add throttle, D-Jet doesn't get the fuel flowing quite fast enough, hence the stumble and die.

The TPS might need attention. It results in more fuel being injected during acceleration. I did a write up on how to dismantle, clean and adjust. It may be here somewhere and is in the Benzworld R/C107 forum EGv107 stickies under Djetronics.

There's a screw on the side of the ECU. It should be set "In the middle" - turn it until your idle starts to slow, then turn the other way and count the clicks until it starts to slow again. Divide by two, set it there. Unless you have a CO meter to "Do it right".

The ECU screw only adjusts idle mixture. As soon as you open the throttle a few thou, mixture control moves to the MPS. It can be adjusted, but not advised without accurate exhaust analyser that can be used whiled driving car or on dynamometer.

There are two adjustable items off your fuel rail. One is a pressure regulator, another is a "Dampener" - perhaps your dampener is adjusted too far in? It's supposed to keep you from being able to hear injector pulses, but if a sudden fuel surge to the injectors causes a drop to 20PSI, it's possible this is too aggressive.

The dampener is not meant to be adjustable (it may have a screw, but that is not intended to be used for adjustment) The FPR is. There is always a large excess amount of fuel being returned to the tank via the FPR and damper. This is needed to control pressure. If ecu calls for more fuel via injectors, then only way pressure can drop is if there is not enough fuel getting to the rail from the pump and therefore not enough going back through the FPR to allow it to control the back pressure.

The dampener is there to smooth out small fluctuations in pressure so injectors don't have a varying pressure upstream that could affect capacity. Not all djets have one.

[Scotto52]

Thanks everyone for your knowledge and insight!

So I guess the first thing to address is the fuel pump (or low pressure). And come to think of it, it is a bit noisy. Lots of talk about fuel pumps here. I found a Bosch pump P/N 69458. Says that it replaces a few different Mercedes and Porsche pumps. Also, different sites says it the correct fit for my vehicle. I think I'm going to get because that's probably not a bad idea to have new. I'll also check for 12V at the pump connectors.

The solenoid for the vacuum advance that I bypassed. What is the correct name for this? And where can I find it? Dealer only? Maybe I'll check Ebay. I'm not a big Ebay fan. I bypassed it because I was not getting any vacuum at idle at the distributor.

As for setting my timing, it was all done at idle, no vacuum at the distributor, gauge in that line, warmed up and in neutral. My dwell meter only goes up to 2,500 rpm's. I need to get my hands on a good timing light, mine is just the basic.

I've been told to never touch the factory presets to idle screws. Lol. But on the other hand, it would be a good idea to make sure its where its suppose to be. I'll keep that as a last resort. I still need to check for continuity at the MAP sensor and make sure it holds vacuum. And I assume the TPS is good because of the 20 "clicks" test.

So it's apparent I have two known issues here. Low fuel pressure on accel and the solenoid thingy. And clean out the LF fender area of any crud.

And I definitely will post the Petronix II set up once everything is running smoothly. One thing I will say now, Accell custom spark plug wires, I would not recommend.

But again, thanks for everyone's time and knowledge!

[Tomguy]

Originally Posted by Graham

Idle timing is supposed to be set with vacuum connected. Manual may say at 5ATDC, but we find that TDC->5BTDC works best. Vacuum at idle retards timing by about 10-12deg, I believe. So, if timing is set without vacuum, maybe it could be set at 8-10BTDC for a start?

We agree on this, and most items. I know spec is 5 after, with the retard, but 7 to 10 before is a good number without it. I do also think that on these 4.5s that setting timing at RPM and adjusting idle speed after if needed results in a better driving experience, versus setting at idle especially if your mechanical advance is worn and thus doesn't have the travel it should because it's already advancing at idle.
The dampener was just spitballing at perhaps another potential cause is all, sure it probably isn't it, but it doesn't hurt to mark where it is, give it a few turns, and test again.

As for never touching the ECU idle speed mixture, I wouldn't advise it until you've seen your PSI at steady 30-32 PSI and not dropping, and having no other issues. It won't hurt anything (again mark and turn, grease pencils can be your friend).

[Graham]

Originally Posted by Scotto52

Thanks everyone for your knowledge and insight!

So I guess the first thing to address is the fuel pump (or low pressure). And come to think of it, it is a bit noisy. Lots of talk about fuel pumps here. I found a Bosch pump P/N 69458. Says that it replaces a few different Mercedes and Porsche pumps. Also, different sites says it the correct fit for my vehicle. I think I'm going to get because that's probably not a bad idea to have new. I'll also check for 12V at the pump connectors.

The only pumps that are suitable, are the Nissan pump we recommended and the pumps that Pelican lists for the 72-75 450SL. Do not use a pump intended for a later model.

The solenoid for the vacuum advance that I bypassed. What is the correct name for this? And where can I find it? Dealer only? Maybe I'll check Ebay. I'm not a big Ebay fan. I bypassed it because I was not getting any vacuum at idle at the distributor.

MB just call it a valve. Current part number 0015408697 (previously 00 or 70 instead of 86)

Did you have vacuum after you bypassed the valve? Sometimes the port in the throttle body is blocked and needs cleaning with a piece of wire. If you had vacuum up to teh valve, but not after, perhaps the tubes were connected backwards. Try reversing them.

As for setting my timing, it was all done at idle, no vacuum at the distributor, gauge in that line, warmed up and in neutral. My dwell meter only goes up to 2,500 rpm's. I need to get my hands on a good timing light, mine is just the basic.

Presume the car doesn't have a tachometer. Some dwell meters give incorrect readings, but if yours reads about 700rpm at idle, then just rev up to 2500rpm. If you see about 25BTDC, that should be about right. It should drop to about 7-10 BTDC at idle without vacuum. Withthose numbers, timing is about right.

I've been told to never touch the factory presets to idle screws. Lol. But on the other hand, it would be a good idea to make sure its where its suppose to be. I'll keep that as a last resort. I still need to check for continuity at the MAP sensor and make sure it holds vacuum. And I assume the TPS is good because of the 20 "clicks" test.

There is no problem at all in adjusting idle rpm. It is the slotted screw just near the AAV. If you screw it all the way in, engine should stop (if AAV is not leaking) Adjust for idle at 700rpm - usually about 1 1/2 to 2 turns open. 20 clicks on TPS is good, but don't assume anything The TPS does control additional fuel needed on acceleration.


[/QUOTE]

[Graham]

Originally Posted by Tomguy

As for never touching the ECU idle speed mixture, I wouldn't advise it until you've seen your PSI at steady 30-32 PSI and not dropping, and having no other issues. It won't hurt anything (again mark and turn, grease pencils can be your friend).

You are right, you can adjust the ECU screw as much as you want, but it just makes sense to adjust the fuel pressure first. (I never said to never touch the ECU idle mixture screw.)

I did say to not to touch the MPS adjustment without an exhaust analyzer. That is the main mixture adjustment.

Lets see what happens once Scotto has a proper Djet pump providing 1L/30sec at exit of FPR/damper with engine off. And one that keeps rails at ~30psig regardless of engine rpm and load. No short cuts on this

[Scotto52]

Ok, pump and valve is ordered. I think my buddy can get a hold of a higher end timing light to use. Parts won't be here till next week, which is perfect because its been so hot. I'll keep it updated. And thanks again everyone, stay safe!

[Graham]

Which pump did you order?

[Scotto52]

The one from Pelican that fits a 450SL.

[Graham]

Hopefully a 72 450sl! This Number for the Bosch 0-580-464-069. It will need some extra fittings including a rubber sleeve if original mount is to be used. The Nissan pump is actually a better replacement.