1972 280SE 4.5 Quits when Hot - PeachParts Mercedes-Benz Forum

Fisherman · #1 05-17-2008, 06:50 PM

Help...
Thought I had the 280 sold today until about 10 minutes after the guy left he called to tell me he was on tehside of the road and that the car had quit on him. I couldn't believe it.
Towed it back to my shop and here is what I have going on:
Car starts perfectly after sitting for a bit and then after about 5 minutes or so it starts to run rough and will die unless I put a little throttle to it and keep it running. I can let it idle down but I have to then pump the throttle to get it back up to rpms. I can hold it at around 1500 rpms forever but then as soon as it goes back to idle it dies.
Let it sit for 10 minutes and you can repeat thsi procedure.

I pulled the fuel filter and bypassed it but no change. I pulled the map sensor line and used my mity vac to keep vacuum to it, no change. I pulled the wires off all the sensors (not the primary water temp that kills the engine) one at a time and no change.
Fuel pump appears to be delivering fuel well and at pressure as I got sprayed pretty good bypassing the filter.
I am at a total loss on this one and I still have the guys money but if I can't get this figured out I am going to return the money.
In either case I have to figure this out.

One last item of note. This is the first time I have driven this car with the temperature over 70 degrees (it is 95 here today). Could the heat have something to do with this and if so, I really need help.

Thanks in advance for your help.
Fisherman

jnanasakti · #2 05-17-2008, 07:43 PM

I had a similar problem with my 78 450sl. It would start and run for a bit and then die out with 1/2 hour. After spending lots of money on non-effective dealer suggested solutions, I figured out the problem was 2-fold. 1) "fuel pump relay". On my 78 450, there is no fuel pump relay, instead there is a wire that goes directly from the battery to the fuel pump, which has a 20 amp fuse; that was blown. Once I replaced the fuse, the care stayed started. That was the main problem. 2) Bad air filter, I figured this out when I drove the car without the intake housing connected while testing. Air filter is simple, and I'd guess that isn't your problem, but if you haven't checked.

However, right now I am having an overheat problem which I'm still diagnosing. But this is different, cooling related problem.

...if any of this helps.

Fisherman · #3 05-17-2008, 08:20 PM

Yeah I thought of that too but the darn pump is running all the time so the relay is fine.
Wish it was that easy...

ctaylor738 · #4 05-17-2008, 09:16 PM

Is there a chance that you are only or mostly running on the auxiliary air valve? The coolant warms and it closes and it can't get enough air at idle. Try adjusting the idle screw out.

Do you have continuity between terminals 12 and 17 on the throttle switch with the throttle all the way closed?

Fisherman · #5 05-18-2008, 12:21 AM

I'm not sure this would be enough though since it starts and idles when the vehicle is theoretically hot. I mean coolant at 175. It will idel for like 5 or 10 minutes and then it starts to get rough and then it will die.

As I noted in my other post though if I keep the rpms up it will run okay so I can't imagine it is actually tied to fuel delivery but at this point I will try anything.

Thanks for your help. Any other ideas?

Sensors maybe?

popscat · #6 05-18-2008, 12:59 AM

replace your coil it has outlated its useful life. and your air induction systems will cause this problem if you dont have a nice blue spark from the coil it is weak.

popscat · #7 05-18-2008, 01:08 AM

check your fuel pressure and your crossover vavle the pump gets it grd its grd on the neg side of the coil? my 77 sl450 get it grd on the coil.

ctaylor738 · #8 05-18-2008, 08:10 AM

"One last item of note. This is the first time I have driven this car with the temperature over 70 degrees (it is 95 here today). Could the heat have something to do with this and if so ..."

This statement is what made me think of the aux air valve. The wax bulb and the air slide are not exactly reliable at their advanced age. You may have been running on a combination of idle air and aux air. In the hot weather, the coolant is hot enough that the bulb has finally closed off the aux air and this kills the engine.

Remember, at idle and fully warm, the only place the engine gets air is through the idle air plumbing. You open the throttle a bit, and you start to get air through the throttle.

Anyway, that's my theory. Just for grins, why not give it some more air with the adjusting screw? You don't even have to take the air cleaner off.

Fisherman · #9 05-18-2008, 11:54 AM

Thanks for the ideas. I'll let you know how it goes today.

God I love this forum... and to add to it, I think this is my 1000th post. Probably not a real milestone to most but makes me kind of proud to think that I have hepled as well as being helped.

Fisherman · #10 05-18-2008, 08:39 PM

So checked the idle mixture and yes it was cranked all the way down, unfortunately this did not fix the problem.

More observations though:
The vehicle during the intire warm up period exhibits no sign of missing etc but after about 5 minutes idling after reaching a solid operating temp it starts to happen. You can let the car sit for about 5 minutes and it will fire back up and run just fine then start missing and die again.
Youc an hold high rpms almost indefinately when this event starts and keep it running but if you let it down to idle each time it gets harder to keep running until it finally dies.
Once it starts this dying thing you cna let it sit for about 5 minutes, then restart it and it will run for maybe ten minutes then go though the same porocess and die again. If you try to start it right after it dies it will not start but if you let it sit for 10 minutes it starts like normal. If you only let it sit for say 2 minutes it might start but this missing and then dieing event starts much faster like within 2 minutes.
Guess I never found this before since the car never actually got warm this winter.

Checked the sensors as best I can, when grounding the 212 sensor it does nothing but I don't think that is my problem.

Checked the resistance through the coil between the center post and the connection to the distributor and it appears just a little high at 11.15K ohms. The book says 4K to 10K so maybe but my meter is probably not the top of the line and could be off that much? I have ordered a replacement.

Along with the coil that I ordered I decided to pick up the basics including:
rotor, cap, points, condensor, then go ahead and fis the water pump where it is leaking a little. I may was well sell the guy a solid ready to drive cross country car.

Thanks for your insights. When I get the parts I am going to do aonly one part at a time to see if I cna find out what is causing this and then let everyone know. Darndest thing I have run into....

Thanks

JoeyTheB · #11 05-21-2008, 12:08 AM

Helpful d-jet site:
http://www.icbm.org/erkson/ttt/engine/fuel_injection/d-jet.html

Engine Will Not Start Warm

Check the following: Defective thermotime switch, temperature sensors or high resistance at trigger contacts.

Fisherman · #12 05-24-2008, 11:19 AM

Thanks Joey I'll give that a shot also this weekend.

ctaylor738 · #13 05-24-2008, 04:13 PM

The fact that the idle adjust was screwed down all the way is trying to tell us something. It still sounds to me like it can't get air when it's completely warmed up. You let it sit, it cools down, it gets some air, and starts. Can you verify that it is getting air through the idle valve?

Graham · #14 05-24-2008, 09:50 PM

Quote:

Originally Posted by ctaylor738

The fact that the idle adjust was screwed down all the way is trying to tell us something. It still sounds to me like it can't get air when it's completely warmed up. You let it sit, it cools down, it gets some air, and starts. Can you verify that it is getting air through the idle valve?

I think ctaylor is on the right track. If the Aux air valve gets hot enough to close and the idle screw is all the way in, then the car WILL die at idle (unless there are other vacuum leaks).

I would adjust the idle screw so the car idles at say 1400rpm (but definitely under 1500rpm**) when cold. Then drive until hot and see what happens. If AAV closes, idle speed should drop but car should keep running at idle. If this happens AAV is sticking. Then try easing cold rpm setting down until car will idle when hot at say 800rpm or so. If AAV is sticking, it can probably be freed up by soaking in varsol or even something like simple green - You can also check it's operation in hot water while it is out of car.

** above 1500rpm, there is an automatic fuel cutoff (I think) - this causes the engine to surge up and down in rpm - need to keep idle rpm below 1500rpm to prevent this.

Good luck.

mercmad6.3 · #15 05-25-2008, 08:31 AM

Do it the right way,
1/Get the ignition Perfect.
2/ Replace the filter and the hoses next to it at the tank.
3/ DO A PRESSURE TEST!
If you have perfect ignition the fuel will burn.
If you have clean fuel ,it will burn
If you have pressure the injection will work.28PSI minimum,'getting sprayed 'is not testing the pressure.
By mucking about doing things in the wrong sequence you are wasting time and energy.
my own thoughts are that the coil has gone.
Do you know there is a resistor in the rotor?,this will also cause the engine to break down when hot.
Pressure testing will also indicate a blockage at the tank return line( very common ,too high pressure) or a Worn out pump causing low pressure which ceases to flow when hot and blocked filter which will cause the pressure to fall as the car is run ,to the point where it actually stops.