SL Hesitant Accelleration for first 15 minutes, once in a while
 

Hi gang,

Can anyone take a guess at why every once in a while my 74 450SL runs really rough with poor accelleration and pushes out fuelly smelling smoke while under 2000 RPM? Right as she passes 2000 and above, she smooths out but still has weak and bumpy accelleration. She never thinks of stalling, just a rough and weak gas pedal. After 10-15 minutes, the symptoms gradually go reside and by the time I get home, it is purring as usual. This happens maybe once every other week and does not seem to be outside temperature dependant. I have checked the cap, rotor, plugs and points and variouse wiring, but eveything seems ok. I've made a cursory check for vacuum leaks, but the hoses and connectors seem OK, too. I do not know the history of the injectors/seals, but read somewhere that injector weep might be a suspect. What else might cause this behavior???

I inherited this car 6 months ago and this problem is apparently a new issue and may be worsening. I called the cars old merc mechanik (in her old home area in Socal) and he told me he had given very thorough maintenance to this car and he did not know might be suspect. But then he is 1000 miles away now.

Rather than drop her in the lap of some local mechanic with no diea of where to start looking, I would like to try to offer him some possible angles.

Anyone what to take a shot here??

You might check the thermo-time switch and the cold start valve. The switch is supposed to shut off the valve after a short time. It may have failed. If it is OK, then the valve may be leaking a bit of fuel when it should not. You might also check the cold start relay, it is rely #2, the first one on the top on the right in the relay stack. They are located up beside the fuses in the passenger footwell. All this is described in job # 07.4-125 in the CD ROM manual. The manual is cheap, $19.99, from MBUSA; check their classic technical literature section if you do not have it already.

230/8

captain scott
 

i had a problem like this with my 380 and found that i had a vacuum leak around my fuel injectors replaced them and the problem went away. try spraying a little water around them with the engine running and if you have a leak you will see the engine almost die.

thanks cclake. That sounds like an easy initial test. I'll start there tonight.

you could have a little gunk in the fuel system, or a slightly clogged injector or two. try a bottle of Techron in the tank.
fuel accumulators wear out, too. if that's never been replaced in your car it's a good and easy dyi $100 part.

My 560SL had basically the same symptoms. What threw me off was that the rough running stopped after the car was fully warmed up. After messing with the mixture stuff for a while to no effect, I replaced the dist cap, rotor, wires and plugs. Problem solved.

Herr Fuchs, mein Mekaniker, said that he has seen many ignition problems cause this - that during warm up, everything needs to be perfect, but at operating temperature, marginal components will still work.

Thanks for the ideas.

I didn't yet check the injectors per cclake, but will tonight. Chuck mentioned plugs/wires/cap/rotor, but all looks fairly new there. I may just bite all of those off and replace them to rule that out, too.

Last night I pulled off the cold start valve and aux air control body. The Cold Start Valve appears to be brand new (car is new to me since Feb) but I didn't check the electrical side of it. The springloaded air checkvalve that is hosed to the right valve cover was not sealing well at all and so I cleaned it and the air passages of gunk. It sealed nicely after I was done. And so I put it all back together and thought I found the smoking gun.

Today it ran great in the AM and then at lunch it ran like dogdoo again. Rough idle, gassy smelling exhaust, rough/weak accel. that gets better but not perfect over 2000 rpm. It did this for 15 minutes (well past warmed up) and improved a little but still is not right. So what else can I check? It sure seems like a rich condition, but what can I do to verify it? If I disconnect the plug to the CSV, might that rule out richness from that component? The aux air valve housing didn't seem to have any more moving parts (except the idle screw) so I don't think that is the culprit. Would the fuel accumulator have eny effect like this? Or is this all an intermittent bad Ignition Control Unit having a field day with the CSV and injector timing, etc?

I am so frustrated right now. Smelling gas and having poor accel/idle is just about what I expect every time I start her. It's a conditioned response... that I would like to make go away, but it will take some time after/if I ever fix this thing...

Oh, also, what fluid will not damage the injector seals if I were to try to test for leaks there? I have some carb cleaner that is VERY flammable and I suspect might do bad things the the rubber seals. Or is water a perfectly OK test here? reccos?

Thanks for your help and patience.
This forum has an excellent group of followers.
Scott

OK -

It's a 1974, so it's D-Jet (electronic injection). I am going to put a post on the Vintage Forum, where we have several D-Jet experts.

Have you checked the wire going into the air filter horn?
This is the outside air temp sensor. If this wire is broken or disconnected, it will think it's REALLY cold outside and will make your engine WAY too rich. This is quite often a problem on rich-running cars.

If you disconnect that, and it still acts like crap, that's it. If it runs worse, or if you see the idle speed change, then it's probably not it and you can move on.

Timing should be 5° ATDC WITH the vacuum connected. Some mechanics ignore that last part and set it to 5 with it disconnected. That will kill your power and make it run like crap. Also, if you've been fooling around with the points, you WILL need to re-time your engine. Changing dwell changes the timing.

Other than those simple things, when it's still cool and running like garbage, shut it off and pull 2 random plugs on different DJet circuits (not 1 and 5, or 4 and 8, or 6 and 3, or 7 and 2, but like 1 and 4, or 1 and 2). If the color isn't uniform, or if one has gas on it and the other doesn't, pull the other plugs. It's possible you have dirty trigger points. This can cause way too much fuel to be poured into the cyls, as I've had happen to me before, and the car runs like absolute garbage until it's warmed enough to thin the oil film out. It would repeat when the car sits for a while to cool down. Fix for this is to pull the dizzy, then take the trigger points out and clean them off with a paper towel to remove heavy oil deposits, then a business card between the points to remove the last bits of dirt and oil. NEVER SANDPAPER!

My sense is that this is an intermittent electrical situation. Follow the harness from the brain box under the dash forward to the injectors and the manifold pressure sensor.

Unless the hose is brand spanking new, replace the hose from the sensor to the manifold. Make sure it's clamped on both ends.

What you really, really want is the Bosch diagnostic box hooked into the circuit, so that when it gets wierd, you can spend 10 minutes doing the diagnostics on the side of the road. Would look silly on I5, but it would get the job done.

Lastly, use gumout or the like for the leak test, not water. It's designed for this sort of thing.

-CTH

A wire in the car's engine wiring harness could have a break in it due to the age and vibration. Electrical contacts could be dirty and need cleaning and various termp sensors could be out of range. They all need to be checked.

This might help.

http://handbook.w116.org/engine_74_l.htm

The engines are the same as is the FI system.You want 07.4 engine electrical.

Good luck.

I just noticed it also has your car info, model 107.

Smoking Guns
 

Again, thanks for all the feedback. I've been chasing this for months now and it is seeming to get worse.

Imagine this:

At the end of the work day yesterday (Wed) I started her and she ran like junk. In my nice clothes, I decided to have a peek under the hood and try to pull the temp sensor that goes the air intake horn (made no diff) and then the plug off the cold start valve (also no diff.) I saw the alternator plug hanging at an angle so I re-seated it - the center brown ground wire fell out of the connector. OK, so that had fatigued to the failure point! Perhaps that will have something to do with my jumping tach readings (maybe not...) So I looked further while she is warming up...

Yep, I looked at the thick wire lume that passes thru the firewall from the ECU into the engine compt.

Are you guessing correctly yet??

Yes, the metal standoff fitting that the lume sits in as it passes over the aft end of the right-hand valve cover has NO RUBBER pad or any insulation bewteen the metal edge and the single layer of electrical tape that encircles the lume. I didn't want to disturb it any more than necessary because I wanted to drive home still! But I moved it off the metal stand-off and the engine started to change idle speed and roughened then smoothed out and back an forth for a few moments.

I didn't dig in there yet since I got home late (and took my truck to work today,) but I know some of those wires on the underside of the lume had to be shorted together (or cut completely) by that metal rest. Tonight I'll invesigate and maybe take picture or two for your enjoyment.

Maybe this is not the source of every odd behavior by my car, but has to contribute to a lot of it. LOL

Finally, I've found something wrong!!! Now I have to track it all down and repair it. I'll still be following everyone's advice to check the trigger points, timing, sensors and the never-ending search for vacuum leaks.

More tomorrow.
Scott

Todays the day
 

Thanks for all the ideas.

Today is the day I am going to dive under the hood.

I didn't see any broken wires in the ECU lume, but I'll find the problem. I am going to ohm all the leads.

More tonight with pics...

Scott

Not quite yet
 

Sunday I did lots of checked and fixed the below noted stuff and she ran better than ever. But she still ran junk this AM. I am closer now, though...

The harness from the ECU checked out. Every lead ohmed out ok, with no bad connectors to. There are 3 leads that go to relays or something in the right footwell that I didn't try to pull (yet.)

Used Gumout to leak test all of the injectors, proved nothing. No idle changes after spraying each base.

I put zip ties on all vacuum hoses to ensure they were not sucking by. One might have been suspect.

I did find the (not sure what it was) large 1" collar nut that holds a 1/2" metal tubing that passes down to a vacuum device on the right-front of the block below the alternator was VERY loose.

In the valley, right side aft, there was a 1/4" vacuum nipple that had a hard hose on it that was cracked. It went to some sort of check valve and then via the usual clear hard line down to something below the alternator.

The cold start valve did't spray anything on start, nothing. It was around 75F, so maybe it wasn't supposed to... I need to check furhter here when I get the specs.

Air temp sensor didn't do anything to idle when I pulled it, so am not sure it is OK.

Still pending ideas are:
Air temp sensor
engine temp sensors (2 that I saw, one aft and one fwd)
fuel accumulator
fuel pressure reg
cold start vave (even though it looks brand new)
cap and rotor (many comments here to just change them - they lool new though)
plug wires (same story)

So my parts bill is going to increse, but I am not giving her to the shop until I have all these "nickle and dime" repair items that could be contributing, verified ok or changed and verified ok.

S

hang in there. your methodical approach is certainly inspiring me, and
more importantly, shows you are going to solve this.