88 300E low idle problems after fuel pump fix

[benzborg]

1988 300E (124), 200K
recent low idle(to stall) related to fuel pump repair problem so far:

Car breakdown leads to re and re of fuel pump, no gas being pumped through line to engine, we assumed FP was to blame from square one.

Cleaned contacts and test. Still no fuel thru line, can't hear pump.
We then checked the FPR (fuel pump relay) and find a bad cold solder connection on board. Repair solder connection as per this being a common problem for these FPRs according to Google. Fire it up - car starts fine, good pressure. Shut it off. Change fuel filter and put it all back together.

While putting it back together, I noticed and replaced a burnt fuse in the OVP. This was likely burned out for the 2 yrs I've owned the car. It looked very old, not a new burn. ABS lite never came on so who knows...not I.

Anyway, Fired up fine again, go for test drive - notice low idle and car will stall at a stop w/o a little foot on the pedal or putting car in neutral. Worse with load like headlights and heater. Checked alternator and VR - 13.6 volts. So that is rulled out.

Replaced FPR,(yesterday) no improvement in idle speed..
Replaced OVP (today), still no improvement to idle still.

I'm almost thinking that removing the good fuse from the OVP might bring my idle back up to 700 like before. There can not be *anything* else that caused that RPM to drop from 7/800 to 4/500 other than that fuse being replaced. (AFAIK) I read about a "mechanical" rpm level kicking in around 500 if the electronic end is not working correctly, and suspect that is what is going on here.

Removing the fuse defeats the OVP however, and still no idea why that fuse was burnt out from the onset of all this. The car was likely driven with that burned fuse over the last two years since there was never a reason to check it. Fuel pump was always fine. New day tomorrow, I'll pull the replaced fuse in the OVP to get a final ruling. If this turns out to be the case, this car runs better w/o a fuse in the OVP which would tell someone who really knows their Benz(s), what is really going on behind this low RPM deal.

Any ideas what the heck would be causing the change in RPMs from 700 down to 4 or 500 hundred now, since we did not touch anything but fuel pump, fuel filter, FPR and OVP? The low idle came immediately after replacing the burnt fuse in the OVP.

Believe I've used up all my "get out the dealership" free cards and may have to hand it over for the high dollar diagnosis.

--
why me?

[Ferdman]

Recommend pulling the distributor cap and checking the condition of the terminals and the distributor rotor. Also, check the condition of the sparkplugs and ignition wires.

[lorainfurniture]

Did you replace ovp with a new one?

Also, check the idle switch located by your throttle linkage. Sometimes, the wheel on the switch breaks, and the car does not go in to idle mode.

just for the heck of it, check your hoses coming to and from your idle air control valve.

I had a similar problem before, but the ovp was the fix for me..

[benzborg]

Originally Posted by lorainfurniture

Did you replace ovp with a new one?

May as well be, one that works well in a similar low mileage 300E which runs like a dream.

Also, check the idle switch located by your throttle linkage. Sometimes, the wheel on the switch breaks, and the car does not go in to idle mode.

I'll have a look. Being new to repairing the car I wouldn't have known to check it. The car does seem to jump in and out of the good idle range, it may stay around 600 a second or two just before full stop then progressively goes down until it wants to stall. Only at full stop all this happens. As long as the wheels are turning even with no foot on the gas pedal, it won't try to stall. Since we never went near that idle switch, I'm wondering why this still or exclusively seems to be related to changing the burnt fuse in the OVP. A faulty OVP is supposed to make the ABS light come on I understand when it fails, and mine never has or did.

Any idea what would blow the fuse in an OVP in the first place since I have no way of knowing how long it had been burned out for sure, if it had been faulty for the two yrs I drove w/o problems or 'idgit' lights coming on, there was no reason to check that. When the car broke down and completely failed to start because of the cold solder joint finally letting go in the FPR, not knowing that was the problem I cranked and cranked the engine over trying to restart since I was stuck at an intersection. I'm wondering if the long period of cranking would have caused it to burn that fuse in the OVP?

As I said I'm about to go out and pull that fuse and see if my idle returns to where it was before replacing. If it does I'm no closer to the solution but at least the car would idle and not continually try and stall.

I will check the hoses on the idle control valve while I'm at it.

Thanks for the replies, also to the reply previous re wires/cap/distributor - that's all good in there.

Keeping a close eye on this thread since the whole mess has got me somewhat obsessed

[lorainfurniture]

ovp fuse would be caused from something as simple as jumping a dead battery via cables, or the boost setting on a charger.

[Gilly]

Yeah lots of things can blow that fuse. Jumpstarting is a biggie, especially if a high demand consumer is left on, like the headlight, and the jumper wires are connected with the consumer still switched on. A normal jump on a cold day can even do it, because the starter has to work harder. Some dipstick mechaninc might be able to pull it off without these conditions being present, damhik.

In regards to if it's been blown for awhile, one big indicator is how well it ran when cold, as the car is designed to run well when WARM if that fuse is blown, but runs cruddy when cold, hard to start and keep running. Also the idle control valve is in a fixed mode w/o the OVP fuse, but normally will idle.

In regards to no idiot lights on but the OVP fuse maybe was blown, it sort of depends on the car. BIG question is if any of the idiot lights in question would come on when you turned the key to the "on" position. If they didn't, it may have some blown bulbs??? Usually the indicators are SRS and ABS.

Try this, start the car, warmed up engine, pull the connector off the idle control valve and see if the idle changes, and if not can you hear the valve clicking. If you hear nothing, check for power on those 2 wires going in to the idle valve, one should have 12v power, the other is a ground wire, which is controlled by...oh hell, either the ECM or a seperate idle speed conroller, forget which now. The power is coming from the OVP. If no power is there, the OVP you installed might be "close but not close enough".

As an aside, you can just jump power to the fuel pump using 2 existing connectors at the FPR socket, no need to buy a FPR just to diagnose. You can just use a jumper wire as a test. You can't leave it like that though as it is not switched, the pump will run contantly. You can DRIVE it like that, but not a "repair". Just to test or to get the car home or to a shop.

Gilly

[benzborg]

Originally Posted by Gilly

Yeah lots of things can blow that fuse. Jumpstarting is a biggie

Ok. Lots of cold start cranking and jumpstarts last winter likely did that. See below.

In regards to if it's been blown for awhile, one big indicator is how well it ran when cold, as the car is designed to run well when WARM if that fuse is blown, but runs cruddy when cold, hard to start and keep running. Also the idle control valve is in a fixed mode w/o the OVP fuse, but normally will idle.

My reply may be rather long to cover all the ground...apologies for taking up your time.

Gotcha though as per fried fuse = helll to start all winter - but once it warmed up no problem, ran/runs smooth. So yes you're reply tells me that fuse was shot ages ago. ~But~, surprise surprise, I just took the fuse out of the OVP just now as I mentioned I would above in my reply yesterday, and got my 200 RPMs back! (running/limping)


Problem not solved (truly) but my car is back the way it was for the last two yrs I've driven it. I can live with it since it's always been this way. I would however like to get to the bottom of the low idle and get it all working just right because I intend to sell it and get my better condition 300E that I recently purchased on the road in a month or so.

Anyhow.,...

Without the OVP fuse I have my RPMs back, I can turn everything on and there is no worse lowering of RPMs due to power being taken away from that circuit. The downside if I leave it like this, there will be hard cold starts again.

But it's back to "normal" as far as me being used to 600RPM at idle once warmed up, no more near stalls at every red light or in traffic and with summer now close - I've got more time to sort the low idle problem out. I use the car daily and driving around worrying about a stall every stoplight or turned corner was a bit too much on the nerves. Removing fuse as an experiment clears one whole side of the problem up. It's not the FPR or the OVP or the fuel pump. So now I move onto your suggestions below.

In regards to no idiot lights on but the OVP fuse maybe was blown, it sort of depends on the car. BIG question is if any of the idiot lights in question would come on when you turned the key to the "on" position. If they didn't, it may have some blown bulbs??? Usually the indicators are SRS and ABS.

Always come on when ignition first turned on, bulbs are there. - never came or come on while running. Why is a mystery still. Two working fused or OVPs gave similar results, good starts but low idle RPM. No idgit lights come on after car starts with _or_ w/o fuse. Only light I've ever seen come on at the outset of this repair was the "check engine" light, which comes on when the O2 sensor isn't right (apparently). My car wouldn't pass an emissions test this year for my plate sticker so I know there are issues in the exhaust system. As an aside, in Canada, $500 bucks buys a "smoker" car a "conditional pass". If anyone can explain to me how that leads to a cleaner environment please make me the first one to know. Only in Canada eh?

Anyway, that Check Engine light only came on one or two times when we first fired up after repairing the FPR solder connection - and hasn't returned since. It was likely unrelated, or the lousy catalytic converter smoked up with a rich fuel mixture after repair and there was not enough O2 present for that short period. It hasn't come on since FPR repair two weeks ago.

While I'm here on that subject anyway, I might add that a duty cycle test shows that engine running on the rich side. Gas mileage isn't the greatest either. The guy testing said all other readings are right so any adjustment would throw the right ones off for me to get a leaner mixture. Six of one - half dozen of the other, so we didn't adjust the cycle with the allen wrench.


Summary. Runs rich with or w/o fuse in OVP. No idgit lights come on with or w/o that fuse. Engine runs with low RPMs (4/500) with fuse, normal 6/700 spec idle RPM without fuse.

Try this, start the car, warmed up engine, pull the connector off the idle control valve and see if the idle changes, and if not can you hear the valve clicking. If you hear nothing, check for power on those 2 wires going in to the idle valve, one should have 12v power, the other is a ground wire, which is controlled by...oh hell, either the ECM or a seperate idle speed controller, forget which now. The power is coming from the OVP. If no power is there, the OVP you installed might be "close but not close enough".

Since I tried two, now three different (working fine in other 88 300Es)
OVPs ending up with similar lower idle RPM results - could it not only be that the idle control valve needs replacing? I do not hear anything since the engine is a little noisy running and I'm not sure, having never heard the sound, what to listen for exactly. I'll pull the connector tomorrow in daylight and see if idle changes but assume I'd have to put fuse back in OVP for that test?

As an aside, you can just jump power to the fuel pump using 2 existing connectors at the FPR socket, no need to buy a FPR just to diagnose. You can just use a jumper wire as a test. You can't leave it like that though as it is not switched, the pump will run contantly. You can DRIVE it like that, but not a "repair". Just to test or to get the car home or to a shop.

Yep, we did some of that on our first pass with the FPR that had the bad solder connection. We jump tested which told us that the FPR was faulty and it was the reason for the total breakdown initially because what remained on contact on the bad solder finally let go and all power to fuel pump quit. That's now behind us. FP and FPR works great.

The car runs fine as I said above but it would be nice to nail down what is really causing the low idle so I can put the fuse back in the OVP and get those easy starts in the morning again.

Hoping whatever is causing the problem, whether idle control valve or other control, is also causing it to run rich as well. Likely wishful thinking but if this tells you anything, in that a faulty idle control valve also results in a poor lean/rich mixture, do let me know.


Again ... thanks a lot for your time... what a great resource having some guys with experience at the other end of this computer.

[JohnM.]

Sounds like you have a combination of vacuum leaks, along with a possibly a poor ignition system. These ignition systems seems to wear cap and rotors faster than any ignition system I have had personal experience will. You can easily check their condition visibly by removing the plastic ignition cap cover at the front of the motor. There are 3 5/6mm allen screws holding the distributor cap in place. Especially if the rest of the motor is not in good tune, a bad cap/rotor will just make things even harder. Also changing the spark plugs with proper copper core plugs (NO PLATNIUMS) usually makes a world of difference. All of these things are free/very cheap to check or replace.

To check for vacuum leaks, remove the air cleaner and grab a can of Carb Cleaner/Starting Fluid and start spraying around vacuum/rubber lines WITH the engine running. Pay careful attention to changes in engine RPM to locate the source of any potential vacuum leaks. Fix as necessary.

After all of that, we can start looking at exactly how rich the fuel system is operating. All you will need is a simple volt meter to check this. If you are looking down at the engine bay, standing on the driver side, there should be a round plastic unit with a screw-on cap. Unscrew that cap and you are presented with 11 different numbered pins or sockets. This is the X11 diagnostic plug and it can tell you a lot about how the engine is running. Plug your negative lead into the #2 slot, and your positive lead into the #3 slot. Set your VOM to read DC voltage. Turn the key to the "on" position, which is the last one before cranking. Note the voltage. Now start the engine. Your voltage reading will be a fixed number while the engine is running in "open loop", once the engine enters closed loop, this voltage reading should start fluctuating, fairly rapidly. Note the voltage. If this voltage does not fluctuate or fluctuates slowly, there are problems/faults that need to be addressed somewhere else in the injection control system. Now revv the engine and hold it around 2500rpms, and note the voltage reading.

Once you can gather a little information with your voltage meter, it can tell you (almost pinpoint) the source of any problems. Good luck.

[Gilly]

Just a quick hit and run here, getting ready for work; MY experience is that a vac leak will cause high idle speeds, not low.
The noise in the control valve is just like a relay clicking.
If you want to throw a part at it instead of diagnnose, I'd say a idle control valve. But I'd rather see it diagnosed....
Gilly

[benzborg]

Will try those diagnostics very soon and be back with the figures. Memory not being the best IIRC the reading for lean/rich was ridiculously low. We tested at a garage close by using a friend's multimeter since mine is nowhere to be found. I'll call him tomorrow and retrieve those numbers.

Brand new set of H9DC Super Bosch plugs, wires not bad and lots of contact left on distributor cap. Always ran smooth. I will have another look at the cap since it's been more than a year since changing.

Thanks for the good info John (et al)

back shortly.

[benzborg]

Originally Posted by Gilly

Just a quick hit and run here, getting ready for work; MY experience is that a vac leak will cause high idle speeds, not low.
The noise in the control valve is just like a relay clicking.
If you want to throw a part at it instead of diagnose, I'd say a idle control valve. But I'd rather see it diagnosed....
Gilly

So would I see it diagnosed having just checked the price for the part - $300 minimum according to Google hits. Ugh

Serious robbery methinks.

[Cal Learner]

....And please don't leave the fuse out of the OVP. That puts a lot of very expensive electronic components at risk. It's called an overvoltage protection relay for a reason.

[Gilly]

AFAIK, once the fuse is blown, the parts protected are not in any jeopardy. The blown fuse IS providing the protection. Or it was.......hard to explain. Sort of like a circuit breaker in a house, once it trips. Except in this case the components are not functioning. The engine, believe it or not, can run even without the engine control module (on a CIS injected engine), but are in a set mode, called fixed operating mode.

Gilly

[benzborg]

Originally Posted by JohnM.

After all of that, we can start looking at exactly how rich the fuel system is operating. All you will need is a simple volt meter to check this. If you are looking down at the engine bay, standing on the driver side, there should be a round plastic unit with a screw-on cap. Unscrew that cap and you are presented with 11 different numbered pins or sockets. This is the X11 diagnostic plug and it can tell you a lot about how the engine is running. Plug your negative lead into the #2 slot, and your positive lead into the #3 slot. Set your VOM to read DC voltage. Turn the key to the "on" position, which is the last one before cranking. Note the voltage. Now start the engine. Your voltage reading will be a fixed number while the engine is running in "open loop", once the engine enters closed loop, this voltage reading should start fluctuating, fairly rapidly. Note the voltage. If this voltage does not fluctuate or fluctuates slowly, there are problems/faults that need to be addressed somewhere else in the injection control system. Now revv the engine and hold it around 2500rpms, and note the voltage reading.

Once you can gather a little information with your voltage meter, it can tell you (almost pinpoint) the source of any problems. Good luck.

Key in the on position reads proper spec - 12.4 I believe? When I start the car we got a reading of 2 volts at idle. A mechanic close to home has done this test a couple of times. The spec I believe he said was around ten? I know that's a very rich reading. He has a similar 300E giving approximately
the same low reading but in spite of this his and my car run pretty well. Fuel economy suffers but oh well.

My other thread about the misfire I've developed (smoke no fire) aside.
I'm getting up to speed with all this and your feedback, and as you know I'm going to sell the car but I hate to have to "give it away" if I could do some low dollar repairs making it good value for the money to a buyer that can carry on with more extensive repairs/upgrades. Having checked, cleaned and WD40'd all the sensor plugs, FPR and OVP I've pretty well done everything except the O2 sensor. The low idle is indeed better now. We tinkered with the lambda but didn't have much luck getting better readings without throwing performance further off, the fluctuations weren't showing the numbers we were looking for.

[slk230red]

Originally Posted by benzborg

Key in the on position reads proper spec - 12.4 I believe? When I start the car we got a reading of 2 volts at idle. A mechanic close to home has done this test a couple of times. The spec I believe he said was around ten? I know that's a very rich reading. He has a similar 300E giving approximately
the same low reading but in spite of this his and my car run pretty well. Fuel economy suffers but oh well.

My other thread about the misfire I've developed (smoke no fire) aside.
I'm getting up to speed with all this and your feedback, and as you know I'm going to sell the car but I hate to have to "give it away" if I could do some low dollar repairs making it good value for the money to a buyer that can carry on with more extensive repairs/upgrades. Having checked, cleaned and WD40'd all the sensor plugs, FPR and OVP I've pretty well done everything except the O2 sensor. The low idle is indeed better now. We tinkered with the lambda but didn't have much luck getting better readings without throwing performance further off, the fluctuations weren't showing the numbers we were looking for.

What numbers did you get when you adjusted your duty cycle?
Did you test the 02 sensor?
Did you verify that the Micro-Switch is adjusted and working correctly?