'75 450SL Idle/Vacuum Help - PeachParts Mercedes-Benz Forum

350SL4spd · #1 08-04-2006, 04:47 PM

I just got done doing a front end job on my buddies 450SL (timing chain/gears/rails/tensioner/oil pump & chain/etc/etc; someone waited almost too long to change the chain). She ran like a top before the major surgery. She runs even better now, but lopes @ idle: She'll idle alone perfectly then stumble & miss for a few seconds, then recover & idle fine. Sometimes she'll idle fine for minutes before she stumbles.

I'm betting on a vacuum leak (or intermittent vac leak) somewhere. Where is the best place to look? Or is it more time efficient (in the long run) to just replace all the old vac lines under the hood?

Thanks!
-M-

PS: Can you buy the different color coded vac lines, or do you just replace it all w/ white?

350SL4spd · #2 08-04-2006, 05:39 PM

OK.

A little more testing and I can say that I have the following figures, maybe someone can tell me what they mean:

On top of the vac adv on the distributor I have zero vacuum.

On the bottom of the vac adv I have 8 inches of vac.

She decided to only idle poorly, no good idle, so I couldn't see what the vac did when it idled well.

Suggestions?

Tomguy · #3 08-04-2006, 07:14 PM

You didn't say you readjusted the timing after replacing the chain. That chain also drives the distributor. If it had 6° of stretch on the left (driver's) cam, then it's a fair bet the distributor was set at least 3° more advanced to compensate. After putting the new chain in, your timing is probably late (later timing -> lower idle). Check that first, it may be the easiest (but not most obvious) answer.

350SL4spd · #4 08-04-2006, 08:26 PM

Thanks for the input Tom. I had the whole front cover off, so the distrubutor was reset when the whole thing was put back to TDC & reassembled. The chain had to have close to 10deg stretch, I've never seen one this bad that survived. It made gouges deeper than 1/4in in the head, gouged up the valve covers & timing cover & destroyed a guide & ate a few teeth off of two of the intermediate gears. It was BAD.

She idles great for a time then rather abruptly switches to a low idle/misfire situation.

Can anyone hook me up w/ an engine vac diagram for the DJet 450?

Thanks!

Tomguy · #5 08-04-2006, 08:43 PM

wdba, mine had 12°, but I had no damage due to snapped rails since mine are the old-style metal-backed aluminum ones. I'm sure, if I had plastic rails, it would have snapped. My heads indeed had telltell signs of slap.

Not sure about possible vac sources for the SL, but on my 4.5, there's only 1 line to the distributor (vac retard for idle). Other vac lines not connected to the engine operation but run off manifold vac are the central locks, tranny, brake booster, and MAP sensor. These, as well as the intake manifold seals, are areas where vacuum leaks are possible. Measure your vacuum at idle - it might be anywhere from 14" to 22" at idle depending on the idle air speed screw setting, with NO LEAKS! You want it to be closer to 22", so you may have to close the air speed screw a bit to do this and compensate slow idle with the ECU idle mixture adjustment screw. I've had lumpy idle on my 4.5 before, and depending on how lumpy yours is, it may be just the air speed screw and idle mixture screw need to be synched. If you adjust the one and not the other, you may get a lumpy idle. The idle mixture screw doesn't affect the idle as much until the engine is warmed up, really, because of the function of the AAV.

FWIW, I keep my idle air speed screw open a bit further to keep the immensely large manifold vac (resulting from letting off the gas completely at high speeds) from sucking up tranny fluid and causing billowing white smoke.

psfred · #6 08-05-2006, 09:47 PM

Clean the trigger points and check the ignition points for dirt!

Bad trigger points (or oily ones) will cause a pair of injectors per point set to fail to inject -- lumpa lumpa lumpa......

What condition are the ignition wires in? Oftimes, old wires will work fine until you flex them, and then they crack and start to arc.

Peter

350SL4spd · #7 08-06-2006, 12:31 AM

Peter,

What do you mean by trigger points? In the dist? Plug wires are brand new.

I just got done w/ job # 14-050 testing the vac system for leaks/ switch-over failure and everything met the books specs; My only question is does anyone have the specs for the AMOUNT of vacuum at both dist adv points, etc? The book said they should increase w/ idle, but not how much (or to what).

I've begun to suspect a more serious leak like the dreaded intake manifold leak.... I will test that tomorrow w/ starting fluid.

Mahalo.

autozen · #8 08-06-2006, 10:18 AM

Quote:

Originally Posted by psfred

Clean the trigger points and check the ignition points for dirt!

Bad trigger points (or oily ones) will cause a pair of injectors per point set to fail to inject -- lumpa lumpa lumpa......

What condition are the ignition wires in? Oftimes, old wires will work fine until you flex them, and then they crack and start to arc.

Peter

I'll go along with Peter for starters. Also you should be measuring manifold vacuum. It should be about 17 unches. Forget about distributor vacuum. Once you are measuring manifold vacuum, crank the engine to about 2000 ripums and hold it. Vacuum should go way down and return to above idle vacuum. If vacuum stays low at 2000, you have retarded cam timing. Check your cams again.

psfred · #9 08-06-2006, 11:54 PM

There are four point sets in the lower part of the dizzy (where the large multiwire connector goes) that signal the computer to fire the injectors in sets of two. If one of them is not making contact intermittantly, those two cylinders will get no fuel, hence a miss.

You must remove the dizzy to take the trigger points out. Spray them with carb cleaner while opening and closing them to remove any oil or dirt. DO NOT use any abrasive (practiaclly impossible anyway, as the plastic covers usually cannot be removed by now!). Make sure the carb cleaner doesn't contain methylene chloride or it will melt the plastic covers. Brake parts cleaner will work, too.

My 72 280SE has worn bushing in the dizzy and possibly worn cams, but this isn't really a problem as I have a replacement engine I intend to install someday. I usually only get all 8 cylinder working when the engine is hot.

If you actually have a 76, it has K-Jet fuel injection and this whole discussion is off base!

You should have full vaccum on the retard diaphram in the dizzy at idle with the throttle closed. This vac will drop under load as the throttle is opened, I believe, but I'm not sure what the switchover system is on the 75, it's different than the 72 (temp and AC switch to cancel retard).

350SL4spd · #10 08-07-2006, 11:25 AM

Peter,

It is a '75 D-Jet.

Dist. is brand new (whole dist, not just cap/rotor). Still possible that the trigger points are fouled up? Both Cams are new too & the valve clearances check out. (Methinks that the PO was chasing a problem as well...but it ran great when it came into my shop...)

I have fully gone over all the switch-overs (there are 4 on the last gen D-Jet) and they all check out via job# 14-100 in the sense that they all do what they are supposed to to the idle & vac. I just need to check if I'm getting ENOUGH vac.

Damn do I NOT want to pull the intake off...

Wish me luck.

Tomguy · #11 08-07-2006, 11:44 AM

Have you measured manifold vac? The easiest way to do this is to disconnect the line from the central locks and hook it up to a MityVac. Do it after the check valve, naturally. When the points in my 4.5 are fouled, it doesn't run well either, nevermind just idle!

350SL4spd · #12 08-07-2006, 01:21 PM

Tom,

I just checked manifold vac & I'm sitting @ about 14in at idle. It drops w/ initial throttle, then increases a few inches, but sits less @ about 12-13in @ 2-2500rpms.

The odd thing is she's running good today; none of the "lumpa lumpa lumpa" miss from all day yesterday. I cleaned the ground strap earlier & tightend the new Vac hose on the MAP sensor, but that's all.

She has had one or two brief little fits where she was running like yesterday, but only for a few seconds then back to running fine with a blip of the throttle.

I had suspected the braided fabric vac hose that goes to the charcoal filter of possibly leaking, but it shows better than 15in @ idle.

I'm at a loss. Especially since today she wants to run right most of the time. I'm goingt o double check the points in the new distributor now, though I have no idea why that would cause the intermittent-ness that I'm seeing. Then, maybe, it's just time to replace ALL the vac lines...

PS: Where is the idle mixture screw (so I can see if anyone has been messing w/ it) & do you know what the point gap should be?

PPS: Would the idle mix screw being off from the idle speed screw cause an intermittent problem? I would think not, but...

Tomguy · #13 08-07-2006, 01:43 PM

Well, what can happen are a few different things. First off, if your throttle position sensor isnt set right, you won't have the idle mixture anyway. It's important to make sure it is set right - a simple check is to remove the plug from the TPS at idle, the idle speed will change 99 out of 100 times if it's set right.

If your TPS is set right, if your air speed screw is too far open for example, your idle will be too fast end your engine too lean. But because it's got a low engine vacuum due to this, it will richen the mixture (because of the MAP's feedback) to compensate. If it's in too far, it'll make the mixture richer, but the heavy vacuum will keep it from being too rich - to a point. These two often cause lumpy idle under load where it might be smooth in park, for example. I've found idle to be one of the hardest things to set on my 4.5 PROPERLY to avoid lumpyness. I am resigned to the fact that it'll probably always be a bit lumpy with it set the way I want performance and economy wise.

I don't have my manual with me here (at work) so I can't tell you what dwell should be. I assume you know where the air screw is. Turn it in a half turn (or more) and see if your vacuum goes up (it should be higher by at least 3"). If not you definetly have a vacuum leak. The ECU's mixure screw is "Hidden" under the rubber boot on the ECU, on the side facing the radiator.

350SL4spd · #14 08-07-2006, 01:53 PM

Thanks Tom,

I'll check those things out. I know someone has been playing w/ the idle speed screw, so I have no idea where it's set. I'll turn it in & see if my vac goes up as specified. I also need to check the power locks as a source of vac loss. I know the locks don't work...

I know the dwell angle, I just can't find the gap figure (my dwell meter crapped out over a year ago). I guess I'll just have to replace it now.

I'll update when I try these things...

350SL4spd · #15 08-07-2006, 03:50 PM

Time for a new approach: I decided (after finally getting a visit from my ex-MB mechanic father) that I'm down at least 1 cylinder. The way she lopes & the extra shake (she's really rocking when she's running poorly) that I must be down a cylinder.

Pulled the plug wires off & had spark @ all 8 cyl (boy do I ever

heavy leather gloves and still got bit!). Pulled the plugs out w/ the wires & had spark through them. However pulling the plug wires off had a negative effect on only four cylinders. Cylinders 1, 2, 5, & 7 are getting spark & have good plugs, but aren't giving me any bang. Now for the Aha!: 1,2,5&7 are in a row in the firing order! Not that I'm exactly sure what that means.

.

Tom, you mentioned worn cams in the dis not sending a fuel signal to two injectors at a time. I looked in my dist (which is new) & everything looked good & clean (only two cam lobes in mine?). It sounds like the culprit except for the fact that the car will run great and then get bad, then get better.

Really odd how it keeps running great & then running ****ty for periods. And I have to have it @ least drive-able by 9am tomorrow, as I have an appointment to have the driver's window replaced, because somebody knocked it out....