strange hesitation only while taking off 280se m116 3.5 engine? - PeachParts Mercedes-Benz Forum

body_infinity · #1 12-03-2006, 11:49 PM

Hi all ...I been working through the ignition so all working as it should, I have taken her on the road and now I have noticed that at times (getting more common) that I am getting slight hesitation when I take off after that initial hesitation she takes off very well its almost like a fuel issue but I have changed the fuel filter and still the same? I have not checked the spark plugs but she starts first shot and once over the initial hesitation she runs and works very well, heaps of grunt!

Anyway - Fuel pressure etc has all been tested and is in spec? is there anything else I can check? or can anyone point me in the right direction to start looking? trigger contacts worth a look again??

thanks guys

Tomguy · #2 12-04-2006, 12:15 AM

If you have the hood up and "blip" the linkage, does it seem to stumble for a fraction of a second before going? My 4.5 does the same thing. I blame it on D-Jet's antique ECU and computer control. It can only react so fast - and not that fast. If anyone can help us both out, though, I know I'd be all ears.

las769 · #3 12-04-2006, 03:40 AM

Maybe this will help i had the same problemwith my 300sel 1990
it would stumble of the line once it got past 1200 rpm she ran great
it start right away,Idle good
Turned out to be the coil foud info. here can't remember witch link

body_infinity · #4 12-04-2006, 04:41 AM

Quote:

Originally Posted by las769

Maybe this will help i had the same problemwith my 300sel 1990
it would stumble of the line once it got past 1200 rpm she ran great
it start right away,Idle good
Turned out to be the coil foud info. here can't remember witch link

the coil

hmmm, funny you mention that as I was looking at my coil also as it is an aftermarket red bosch 12v (has 3.5 ohm resistance internal) which I think is too hi for the v8??? anyway if you know the link that would be great as I tried searching this and other sites no go...

I dont think its fuel related or links as all have been checked and seem good..but spark plugs which are not that old I have not had a chance to look at.

as I have said the car seems to idle fine, sounds kinda low but will not cut out, it has good power once over the initial stumble and hesitation, you kinda start to slowly press the accel and she just splurts and is very rough then starts to pickup and normal running.

regards B-I

GaryF · #5 12-04-2006, 05:31 AM

Quote:

Originally Posted by Tomguy

If you have the hood up and "blip" the linkage, does it seem to stumble for a fraction of a second before going? My 4.5 does the same thing. I blame it on D-Jet's antique ECU and computer control. It can only react so fast - and not that fast. If anyone can help us both out, though, I know I'd be all ears.

I cant say I've ever noticed this problem but in theory isnt it the throttle position switch that should give the ECU advanced warning of more fuel being required before the manifold vacumm has dropped. If the TPS was faulty and signaled idle position constantly would this cause the same effect ?

230/8 · #6 12-04-2006, 12:13 PM

Several things to check:

1. Check the points...they can develop a coating which will affect idle quality. MB issued a TSB on this and it is in the service manual. Pitting over time will also reduce the ignition spark energy, even though it is an electronic system.

2. Check the timing...late iginition timing will slow the idle and cause an off-idle stumble.

3. Check the idle speed...make sure it is within specs, should be 700-800 rpm, as I recall.

4. I thought the electronic set-ups used the blue coil? Have you eliminated the points with a conversion kit? If so, it's probably just idle speed and timing.

230/8

carnut · #7 12-04-2006, 09:49 PM

On my 72 280SEL 4.5 there was an idle air screw at the front of the air horn. Turning it out allowed more air thereby raising the idle. Is it too low? The TPS, "throttle position sensor" is under the large black plastic cover screwed onto the throttle body side. A multi wire connector plugs into it. Gently remove the cover and you will see a series of copper lines and "fingers" that brush across them. Clean the copper lines with a pencil eraser and see if its better. Also from memory, I believe you can rotate that switch ever so slightly to anticipate changing the injector pulse width. Without the book, I dont remember the resistance spec? I would think you could mark a point of reference on the cover by the screws and move it and see if it improves, and if worse go back to original setting. My car never hesitated, in fact it was hard to leave a stop light in the rain. The rear tires would break loose! at part throttle. Another adjustment is the manifold sensor on the left inner fender(driver side) at the opposite end of the vacuum hose connection, there is a set screw to move the diaphram within it. Sometimes this screw is covered with a type of wax or a metal cover. Moving this screw ever so slightly will alter driveability. Again mark it prior to turning. Cannt remember which way? And finally there is a serrated knob on the ECU as well to adjust mixture. Best done hooked up to an old school emission or smog machine. Hope this helps. Sold my car 10 years ago!

carnut · #8 12-04-2006, 09:56 PM

I think I still have a theory booklet at home on this fuel delivery system. I'll look at it tommorrow.

body_infinity · #9 12-05-2006, 05:59 AM

ok did my oil check and seems to be good output is good no major heat etc and spark looks strong and to be honest is not that old so next thing I guess is the ECU mixture? I had a look at it today and the nob on the side is all the way to the right

ie I can not turn it clockwise any more so tried the otherway (anti cw) and the idle went slighlty up (only about 4-5 clicks) hmmm could it be running too lean

..I dont get much info on the ecu in my manual

but will try and borrow a co2 meter if I get a chance otherwise will need to play by ear

anytips here??

thanks again

carnut · #10 12-05-2006, 12:58 PM

Well, my memory was close, but not exact. The idle air screw is forward on the intake manifold, next to the aux. air valve. Turning it out will raise idle. 4 to 5 clicks on the ECU knob could be too drastic. Try 1 or 2. I've not read anywhere in this thread that timing was verified at spec. I ran mine 2 degrees advanced from spec. The good thing about this fuel injection system is everything is adjustable, but this is also its weakness, everything can go out of adjustment. Most all the driveabilty issues with this system reside within the distributor. Oil residue on trigger points, distributor points out of adjustment, timing out of spec. To verify the throttle position sensor is correctly adjusted, have the ignit key in the run position, engine off and SLOWLY rotate the throttle linkage. You should hear 20 distinct clicks from closed to fully opened rotation. If during this rotation, you hear a lesser click or no click at all, then one contact is dirty or burnt thru. Also if you hear 18 or 19 clicks then the TPS needs adjusted to return it to 20 clicks. The TPS has 22 contacts, one to signal idle and another to signal wide open throttle. The remaining twenty signal the ECU to cycle the injectors to remain open longer as speed increases. I've seen cases where the car runs better if the air temp sensor is unplugged on the air cleaner inlet snorkle!

Captain Scott · #11 12-06-2006, 12:37 AM

TPS has 22 positions? Yikes!! Sounds like it might be an easy thing to check and rule out. Would this be similar to the '74 450 SL TPS?

Quote:

Originally Posted by carnut

Well, my memory was close, but not exact. The idle air screw is forward on the intake manifold, next to the aux. air valve. Turning it out will raise idle. 4 to 5 clicks on the ECU knob could be too drastic. Try 1 or 2. I've not read anywhere in this thread that timing was verified at spec. I ran mine 2 degrees advanced from spec. The good thing about this fuel injection system is everything is adjustable, but this is also its weakness, everything can go out of adjustment. Most all the driveabilty issues with this system reside within the distributor. Oil residue on trigger points, distributor points out of adjustment, timing out of spec. To verify the throttle position sensor is correctly adjusted, have the ignit key in the run position, engine off and SLOWLY rotate the throttle linkage. You should hear 20 distinct clicks from closed to fully opened rotation. If during this rotation, you hear a lesser click or no click at all, then one contact is dirty or burnt thru. Also if you hear 18 or 19 clicks then the TPS needs adjusted to return it to 20 clicks. The TPS has 22 contacts, one to signal idle and another to signal wide open throttle. The remaining twenty signal the ECU to cycle the injectors to remain open longer as speed increases. I've seen cases where the car runs better if the air temp sensor is unplugged on the air cleaner inlet snorkle!

GaryF · #12 12-06-2006, 08:46 AM

This is the 450 TPS. I think one switch indicates idle, another for full throttle and maybe the other two switches give 8 positions ?

Tomguy · #13 12-06-2006, 10:03 AM

This is a D-Jet Throttle Position Sensor. This one came off a 3.5, but it's the same as the 4.5's as well for reference.

keywords for search: 280SE 280SEL 300SEL 350SE 350SEL 450SE 450SEL

terminal #'s (top down): 9, 20, 12/14, 17.

GaryF · #14 12-07-2006, 04:24 AM

Sheesh, I guess that's too complex to show in a wiring diagram. Wonder why they didnt just put a reostat in there.