1988 560SEC random engine shutdown - PeachParts Mercedes-Benz Forum

Sib · #1 05-10-2009, 08:44 PM

Hello again forum folk back again with hopefully another quirk that your collective minds might have good advice on.

For the last few months the car has on occasion, as in twice a week. Th engine will just shut down. the rpm gauge will drop instantly to zero and the dash lights come up as if the car was in the On position. I do see a check engine light on after the engine dies but I also have always seen the check engine light on if the key is in the On position but the engine is not running.

Things I know about the problem:

1.When the engine dies it will not restart even after many 5 or so second cranks on the starter.

2. The coil wire on the cap has spark during the period that the car refuses to start.

3. I tried turning the key on and off a few times thinking the fuel pump was not spinning. although I can hear a whirrrrr coming from the passenger rear wheel area when the key is put into the on position. It did not seem to help start the car at all though trying to "pump up" the fuel system by turning the key on and off

4. after about a minute or two the car will eventually just start with a roar and resume a 850ish RPM idle in park, And will drive perfectly fine after this episode. ive only on 1 occasion had the car die again just a few hundred feet up the road.

5. when the engine dies it does not sputter then die. its straight to zero rpm as if ignition had been cut. when the engine starts it does not hit a few times then sputter to a start. it instantly starts up and resumes idle normally.

6. Ive replaced all the vacuum lines on the car as well as the spark plugs cap and rotor with bosch stock replacement parts. again though I get spark to the cap but I have not had a chance to see if it gets spark to a plug while the car is in its temp dead state.

7. These cars don't come with cup holders? WTF kind of bull is that? How many times have people spilled their coffie accelerating too fast when the cup is resting in that center console area under the arm rest. AND NO dont tell me you put the seat at such an angle that the arm rest pushes down on the top of your cup.

Hopefully this all is enough info to hear some ideas of what could be causing this. Thanks everyone.

mbdoc · #2 05-11-2009, 08:03 AM

Most likely problem is fuel pressure. That system runs at 100+ psi.

To find the problem you will need a guage that can read that much pressure.

david s poole · #3 05-11-2009, 10:19 AM

if you have spark available when engine shuts down,the next place i would look is the fuel pump relay.while it is unusual for one to misbehave intermittently i saw one two weeks ago.if you keep apair of needle nose pliers with you,the next time it shuts down you can pull out the relay and use pliers to jump terminals 87&30.this will confirm suspicions.

Sib · #4 05-11-2009, 04:33 PM

Thank you both for the info. Ive heard that it could be the fuel pump relay and although ive looked through alot of resources ive been unable to find the fuel pump relay on this car. i did find a unit i thought could be it. it was under a big plastic gaurd along the foot well of the passenger side. i had to remove the carpet then remove this big plastic wall and there it sat up and to the left of a big bright red molex connector right on the vertical foot wall . is this the relay you speak of?

Thank ya kindly again. as for fuel pressure im gonna go pick up a 100+psi meter today and see if i cant get this tested. any perticular area on the system that would be best for testing the fuel pressure?

david s poole · #5 05-12-2009, 10:35 AM

the fuel pump relay sits under the hood about 12 inches from the brake booster towards the centre of the car.there are three relays there.first the ovp relay,then the a/c relay and then the fuel pump relay.you will have to take out the a/c relay so you can get fingers around f/p relay.

Sib · #6 05-12-2009, 05:00 PM

Is this the pig here? if so i took it appart and cleaned the contacts of all 3 relays with small cut up pieces of grocery bag paper. got alot of black dust off of each one and when i pulled the cover off i noticed a strong smell of hot electronics. although upon close inspection nothing looks like its been overheated and broken. the real test here will be to see if the next time it does if jumping the two pins makes the car suddenly start. in the mean time i guess im in the market for one of these relays. TO EBAY AND TO FREEDOM!

Again thank you all for your help you're all champions

tinypanzer · #7 05-12-2009, 05:09 PM

Yikes! I've seen better build quality in a 60's Japanese transistor radio!

I can't believe they made that circuit board out of phenolic, that is clearly a case where fiberglass would be best.

Apart from cleaning the relay contacts which you already did, I would carefully check the back of the circuit board for cracked or stressed solder joints and touch up any suspects...... Just don't put too much heat on the leads for the ICs, they won't appreciate it. Pay special attention to the solder joints on the module's pins.

If you're feeling frisky, replace those electrolytic capacitors as bad electrolytics are a common cause of circuit failure in general......

Sib · #8 05-12-2009, 05:25 PM

Thankfully at work here i have 2 very seasoned electrical engineers that were more than happy to futz with this thing , we checked it out under a microscope and the solder joints look great. they mentioned too that the caps could be bad but after cleaning it and inspecting it i just threw it back into the car. it starts right up ofcourse but we will see if it dies again, i made a jumper for the pins that fits perfectly so all i gotta do is push it in and be done with it.

Thanks for the Advice Tiny Panzer, lets hope my Panzer keeps runnin.

cliffmac · #9 05-12-2009, 07:16 PM

that's electrical/spark/ignition related...a fuel related issue is quick to show up, but not as quickly as you describe...good luck

Sib · #10 05-12-2009, 08:30 PM

Quote:

Originally Posted by cliffmac

that's electrical/spark/ignition related...a fuel related issue is quick to show up, but not as quickly as you describe...good luck

Yeah it seems that the electrical system that controls fuel is whats cutting out so the system probably drops below the required PSI to open the injectors damn near instantly and i get an instant engine shut down instead of a sputter sputter dead.

Thanks for your input

johng99 · #11 05-13-2009, 09:12 AM

My '88 SEC would just die while driving. Lights in dash were functional. Sometimes would start back up quickly, other times it took a minute or two. The intervals between occurances were increasing. A new fuel pump relay solved the problem.

pentoman · #12 05-13-2009, 10:08 AM

Quote:

Originally Posted by cliffmac

that's electrical/spark/ignition related...a fuel related issue is quick to show up, but not as quickly as you describe...good luck

I disagree... i thought the fuel system is so high pressure that the engine instantly dies when fuel pressure is lost. I think this because the rev limiter works by cutting off the fuel pumps.
If you've ever hit the rev limiter you'll know it's a very instant cutting of power.
MB Doc knows his stuff so listen to him. I'd mention that the culprit may be related to the fuel pump relay but not the relay itself - e.g. I believe on your car there's a crank sensor that signals the fuel pump (relay) to keep running. If this signal is lost (i.e. engine dies or sensor is faulty) the fuel pump relay will shut off the pumps.

Sib · #13 05-15-2009, 06:33 PM

Well so far its been 4 days since i cleaned the contacts on the relays of the fuel pump relay. it has not died so far but that does make sense what Pentoman is saying. one day i had taken off from a stop in second gear. i floored it and the car rev'd up and kicked down into first gear. soon as it hit 6K on the guage the engine shut down instead of switching instantly to 2nd gear like it usually does. it started right back up on that occasion instead of being dead for a few minutes...... Hmmmmm well guess we just have to wait to see if it dies again before diagnosis can continue

pentoman · #14 05-19-2009, 02:38 PM

Quote:

Originally Posted by Sib

Well so far its been 4 days since i cleaned the contacts on the relays of the fuel pump relay. it has not died so far but that does make sense what Pentoman is saying. one day i had taken off from a stop in second gear. i floored it and the car rev'd up and kicked down into first gear. soon as it hit 6K on the guage the engine shut down instead of switching instantly to 2nd gear like it usually does. it started right back up on that occasion instead of being dead for a few minutes...... Hmmmmm well guess we just have to wait to see if it dies again before diagnosis can continue

That, for me, seals the deal right there. Because the fuel pump relay is the rev limiter too. And I have been passenger in a 190e where the driver hit the rev limiter in 1st (manual tranny) and the engine totally cut out. It only restarted when we pulled over and messed about in the fuel pump relay area, clearly disturbing it enough to un-jam it. Sounds like the exact same thing you had and a fuel pump relay on the way out.

Sib · #15 05-20-2009, 03:56 PM

Just to update this thread one last time for future forum searchers. It has now been 10 days since I cleaned the contacts on my fuel pump relay using grocery bag paper and have not had any problems with the car dieing. This used to happen at least once every 3 days and now nothing so awesome work everyone on helping find the solution to this problem. I think its safe to say that the gritty carbon and burned dust i found on the contacts of the fuel pump relay had something to do with the fuel pumps getting shut off which would drop fuel pressure and keep the engine from starting.

Also we had fun beta testing the new dyno program for the blackberry storm last night and managed to get 196HP to the wheels using its software.