300SEL 3.5 idle speed...still fighting

[werminghausen]

With the hot weather here in Oman 110+ I am struggling with idle speed.
Engine is not overheating but it is at a limit. Everything is just hot outside and the sun is strong.
Car doesn't seem to like this condition. Hot start can be a drag. But lately the car stalled at a junction with AC fully on etc... disappointing if it doesn't start again.

I tried to keep idle as low as possible and it worked fine until now as the heat becomes unbearable Idle started getting weak...maybe the compressor is demanding more power or something else?

I was testing today and dwell/distr. timing seems to be fine.
I put idle now at 830-850 (hot engine). It drops to 650 when I turn AC on (huge drop) and it goes down to 580 when shifting into gear.


Or the other way around 830 idle
650 in gear and 580 adding AC (so gar or AC are adding about the same amount of restriction)
Anything below 580 would not work and engine stalls.

I don't like high idle when cold...engine is really hunting (1200) and surging and even more annoying is the bang into gear at high idle....my old trans.
for those who know the engine... is there something wrong?
Martin

[Tomguy]

When the engine is NORMAL operating temp (not hot) what does it idle at?

I didn't like the hunting when cold either. I had my mixture and my air screw adjusted just right in combination to prevent the hunt but have perfect hot idle and hot start condition. Of course, when I was driving around in PA the hottest hot start was mid 90's nowhere near Oman temps!

You may need to play with it a bit, or try another aux air valve as yours may not be functioning properly.

Remember, any of the following WILL affect idle speed as well as other conditions:
Dwell
Timing
Mixture (set at MAP)
Idle mixture (set at ECU)
Idle air speed screw
Engine load
Throttle plate setting (MUST close FULLY without binding, NO GAPS; some "mechanics" adjust this and mess up idle!)
D-Jet fuel pressure

Not to mention other things like worn points, a tired coil, worn cap, rotor, etc; I'm presuming your ignition system is up to snuff and it's just settings that need set.

[NutmegCT]

Just a thought: back on our 1961 220S, you could add a solenoid to the throttle linkage, which sped up the warm idle speed when a/c was turned on.


Tom
1960 190b

[Graham]

Originally Posted by werminghausen

With the hot weather here in Oman 110+ I am struggling with idle speed.

Hi Martin,
Do you have your new water pump installed? And a 75C thermostat?

If cooling system is working well, the air temperature is probably less important. But I doubt 40+ year old cooling system can really cope with such extreme temperatures.

You need to adjust idle rpm when car is hot, but it is a fine balance to keep idle rpm high enough when super hot to prevent stalling while still avoiding surging when engine is cold.

You might try tweaking the idle mixture - Not sure what would be best, but perhaps slightly richer on ECU would increase rpm a idle? Sometimes one click on ECU screw will make a difference.

Your conditions are beyond what we ever see.

[werminghausen]

Thanks Tomguy, Nutmeg and Graham

What I experienced...up to ambient temperatures to 100 I had no problem with idles (other than the bangs in the morning when shifting in gear) but now temperatures are just constantly over or around 110F and suddenly a different picture with engine stalling and hard hot starts. I am balancing between hunting idle at start up and stalling when hot w/ AC on.
This is a narrow path and I have no clue how to really solve it.

May be I am playing at ECU but all AFR numbers are good...idle rather on the rich side. Dwell and distributor timing are good. Plugs are good.

Best solution would be what Nutmeg described...lifting idle when hot. That would solve a lot of issues I have.

[Graham]

Originally Posted by werminghausen

Best solution would be what Nutmeg described...lifting idle when hot. That would solve a lot of issues I have.

At idle, the timing is normally retarded by about 5 deg by the vac pod on distributor. This causes higher engine temperature. But when A/C is on, the valve between the vac pod and the throttle body closes and vac retard is disabled.

You might check to see if the switchover valve is there, connected properly and working.

Don't recall how you set your timing, but increasing advance to 27-30 BTDC made my engine run cooler.

[Adv_rider]

Just a thought .. idle compensation valve.... does your engine have one?

I know my W116 with M117.986 engine has one, but I havent searched on the W108 M117.984

[werminghausen]

Originally Posted by Graham

At idle, the timing is normally retarded by about 5 deg by the vac pod on distributor. This causes higher engine temperature. But when A/C is on, the valve between the vac pod and the throttle body closes and vac retard is disabled.

You might check to see if the switchover valve is there, connected properly and working.

Don't recall how you set your timing, but increasing advance to 27-30 BTDC made my engine run cooler.

I don't have any switch over valve at the vacuum line. I think this is stock in my 3.5. Maybe later cars have a switch over valve that disables the vacuum...which makes absolutely sense.

Now the tragic of the day (after I readjusted idle yesterday...I went to 850 for idle without AC/gear) The engine started stalling again and again even if I turned off the AC. The idle is so weak...meaning if engine idles even at 1000 (I turned it up that much today under way) it stalled when engaging the gear...not even speaking about A/C. The idle is turning into a mystery... it is a total different ballgame at these very high ambient temperatures.
Or is there something else? I mean I am disappointed to have this car not functional right now. It runs very well under way but I am busted if there is a stop...stalling comes along with very poor starting after stalling.
I am at a loss.

[Tomguy]

Dripping injectors, including cold start valve. Air intake sensor.

Those are two things I'd check. At 104°F (40°C) the air intake sensor should read 150 Ohms. It should read 210 Ohms at 30°C / 86°F. A huge difference in temp there for only 60 ohms. If you want to experiment, put a straight resistor in there of 150 ohms on the hotter days, IF the sensor is reading over 150 on a hot day like that. If unplugging the sensor while idling does not make a huge difference in your idle the wires may be broken in the plug or harness to the air intake sensor. That'd have you chasing your tail for ages.

[Graham]

Originally Posted by werminghausen

It runs very well under way but I am busted if there is a stop...stalling comes along with very poor starting after stalling.
I am at a loss.

Do you still have your AFR meter hooked up? It would be interesting to know what happens to AFR when you come to stop and car stalls. If you see AFR go way high (16-18+) it would indicate you are starved for fuel. Possibly caused by vapour locking in the fuel rails OR as Tomguy suggested, ECU getting information from sensors telling it to inject less fuel.

Some off the top suggestions to experiment with:

- Advance timing (30degBTDC at 3000rpm?) (should run cooler)
- Try plugging vacuum retard line (stop retard at idle)
- Increase fuel pressure to about 34-35 psig. If that helps, adjust MPS to get CO/AFR back to spec.
- Try disconnecting T1 (temp sensor on air filter)
- Keep fuel tank near full

Some later engines had a fuel cooler installed. It used A/C to cool fuel! You can buy aftermarket air cooled fuel coolers (Summit). May help keep fuel cool in those extreme temperatures.

You did not say if you have water pump and new 75C thermostat installed.

[werminghausen]

Originally Posted by Tomguy

Dripping injectors, including cold start valve. Air intake sensor.

Those are two things I'd check. At 104°F (40°C) the air intake sensor should read 150 Ohms. It should read 210 Ohms at 30°C / 86°F. A huge difference in temp there for only 60 ohms. If you want to experiment, put a straight resistor in there of 150 ohms on the hotter days, IF the sensor is reading over 150 on a hot day like that. If unplugging the sensor while idling does not make a huge difference in your idle the wires may be broken in the plug or harness to the air intake sensor. That'd have you chasing your tail for ages.

Good suggestions with the air intake sensor. I need to measure ohms. I believe that idle goes slightly up when I pull the plug there. Will check.
What does the input from the air temperature sensor do?
It is correcting the idle AFR? Any influence on the AFR while driving?

[werminghausen]

Originally Posted by Graham

Do you still have your AFR meter hooked up? It would be interesting to know what happens to AFR when you come to stop and car stalls. If you see AFR go way high (16-18+) it would indicate you are starved for fuel. Possibly caused by vapour locking in the fuel rails OR as Tomguy suggested, ECU getting information from sensors telling it to inject less fuel.

Some off the top suggestions to experiment with:

- Advance timing (30degBTDC at 3000rpm?) (should run cooler)
- Try plugging vacuum retard line (stop retard at idle)
- Increase fuel pressure to about 34-35 psig. If that helps, adjust MPS to get CO/AFR back to spec.
- Try disconnecting T1 (temp sensor on air filter)
- Keep fuel tank near full

Some later engines had a fuel cooler installed. It used A/C to cool fuel! You can buy aftermarket air cooled fuel coolers (Summit). May help keep fuel cool in those extreme temperatures.

You did not say if you have water pump and new 75C thermostat installed.

hi Graham
I don't have the AFR meter hooked up but I can reinstall relatively easy...10 minutes. Good call
I'll test the temp sensor on the air filter. I saw that the wires look sketchy. Tomguy suggested testing resistance- will do.
Timing: I am running relatively 'advanced': 10 degrees BTDC at idle (w/o vacuum = roughly 0 degrees w/vacuum. I tested 30 degrees at higher speeds and it is true. I don't think tweaking the timing is helping,

My idle speed is not constant or very solid. I'd need to solve this issue.
Together with the bad hot starting it suggests that AFR is too high and engine is starving. I have touched the fuel tank/fuel pump. It is very hot. Around 60 C I'd think. No wonder that they started to cool this fuel tank. The fuel pump is very hot and I fear that the fuel pump is suffering. i heard bad noises from the back.
I have an old fuel pump from a 117 engine. I could install this one if my fuel pump is falling apart.
Thanks, Martin

[Graham]

Martin,
The djet ECU uses the incoming air temperature along with the manifold pressure to calculate density of air. That along with engine rpm is used to calculate the amount of fuel to inject into the cylinders to achieve the required AFR.

Didn't understand the part about your fuel pump, but you do need a proper D-Jet pump, not a K-jet or an aftermarket type. And, it should put out at least 1L/30sec at discharge of Fuel Pressure Regulator. This ensures circulation of fuel back to tank at idle so vapour locking is less likely.

Interesting the 300SEL 4.5 doesn't have the vacuum switch over valve. Must have been something they introduced in 72. In your case, you would use A/C most of time, so why not disable the vacuum retard by disconnecting the tubing at distributor and plugging the tube? That's what the later M117s achieve by closing the valve.

I wish you luck.

[werminghausen]

Originally Posted by Graham

Martin,
The djet ECU uses the incoming air temperature along with the manifold pressure to calculate density of air. That along with engine rpm is used to calculate the amount of fuel to inject into the cylinders to achieve the required AFR.

Didn't understand the part about your fuel pump, but you do need a proper D-Jet pump, not a K-jet or an aftermarket type. And, it should put out at least 1L/30sec at discharge of Fuel Pressure Regulator. This ensures circulation of fuel back to tank at idle so vapour locking is less likely.

Interesting the 300SEL 4.5 doesn't have the vacuum switch over valve. Must have been something they introduced in 72. In your case, you would use A/C most of time, so why not disable the vacuum retard by disconnecting the tubing at distributor and plugging the tube? That's what the later M117s achieve by closing the valve.


I wish you luck.

D-jet pump....upps
My current pump is a aftermarket CIS pump see picture.
There were controversial opinions about this pump when I installed it (my original pump (right side pump of second pic...the one with the 3 ports) was failing and I installed a rel. cheap CIS pump. It worked fine so far but I have now this trouble.
I also have the other pump (second picture left side) It might be from a 117 engine...not sure if d-jet or something else.
I could test and install this pump as an alternative to the CIS pump?

Another thing: I experienced a lot of vacuum when opening the fuel tank at gas station: Is this normal? Is something wrong here. ..However I have seen this for a longer time and never thought about it.

Martin

[Adv_rider]

Originally Posted by werminghausen

D-jet pump....upps
My current pump is a aftermarket CIS pump see picture.
There were controversial opinions about this pump when I installed it (my original pump (right side pump of second pic...the one with the 3 ports) was failing and I installed a rel. cheap CIS pump. It worked fine so far but I have now this trouble.
I also have the other pump (second picture left side) It might be from a 117 engine...not sure if d-jet or something else.
I could test and install this pump as an alternative to the CIS pump?

Martin

The 2nd pic, left side is similar to my 117 Djet pump.

If it is what you need, consider this 280z Fuel Pump Genuine Nissan 1975 1978 | eBay

It's what I'm using on my car and works perfect.. Much cheaper than MBs.