hot idle problem -74 450slc
 

Hoping someone out here has had experience diagnosing this problem.
My '74 450SLC (with approx. 100K on the clock) has a stalling problem after a prolonged run on the highway in hot weather. The engine is NOT overheating, temperatures generally peaking no higher than 110ºC and returning to between 90-100º pretty quickly after exiting the highway. After such a run however, the engine stalls at stoplights when in drive. When in park or neutral, the idle speed is around 850-900 rpm as usual, but dies when engaging drive unless I apply throttle.

The problem does not manifest itself in regular in-town driving (mixed street/hwy). It takes a prolonged run in the highway in 90+ºF heat. The car generally runs well otherwise, just a little shake at idle, especially when cold.

My local mechanic has checked and adjusted the CO at idle (ECU) and load (MAP). I have disassembled and cleaned the auxiliary air valve which was sticky. It now is open when cold and fully closed at around 80ºC. The spark plugs (heat range 9) are in good shape and have a very light tan color on the electrode insulators.

Could this be vapor lock? If so, would adjusting the fuel pressure regulator be a good next step?

Or: bad temperature sensor(s), vacuum leaks, valve clearances too tight? I really don't want to take a shotgun approach to solving this!

My first guess is the aux air valve. They should be fully closed before engine thermostatic temp. If they are still somewhat open at idle engine temp they can close further after the higher heat of the freeway travel. Set idle speed to 850 after its real hot and see if its too much other times.

Another alternative is the mixture is lean and the engine temp sensor is still changing in that temp range. The resistance of the eng temp sensor is logarithmic in its change. Very high when cold above 1500ohms. At idle it is down to 150 - 300 ohms. VW and Porsche used a 200ohm resistor in series to nulify the swings in resistance in the last 20 degrees of engine temp. Adding the resister makes it significantly richer at warm setting (but stable) but makes very little difference in the cold mixture due to the relative size of the two different resistance totals. If the main mixture is now too rich use my basic D-jet mixture correction method at the manifold pressure sensor.

Steve-

Thank you very much for your response and for your invaluable contributions to this forum!

You're quite right, the AAV was the first thing I addressed, it used to not close fully thus causing the idle speed to continue changing in the operating temperature range. The warm idle used to have to be set to ~1300 rpm to be able to idle at all after a hot highway run!

After servicing, the AAV now is open only when the engine is cold and is fully closed by 80°C, checked by putting a thumb over the intake to the valve -lots of vacuum when cold, no suction at all after a little warm-up. Now the idle in neutral is stable at 850-900 rpm after a short warmup, and remains so even after the engine gets hot. Even when I encounter the heat-soaked stalling problem with engaging drive, the idle in neutral is still 850-900 rpm.

I'll try the series resistor on the temp sensor. I take it you mean the coolant sensor, and not the air sensor, yes? Is it reasonable to try simply unplugging this sensor to see if the stall disappears, or does that make it way too rich?

If this resistor corrects the behavior, is this a good long-term fix, or do I need to replace the sensor?

thanks again!

Update:

Well, disconnecting the coolant temperature sensor sure doesn't work (engine way rich, barely idles).

The sensor is at 250 Ohms at ~85ºC. I wired a 150 Ohm resistor in series with it this evening. Seems to run okay this way, no obvious mixture-related issues. Don't yet know about the hot stall, though.

Thinking about it now, it would have been instructive to see how the engine behaves with a short across the sensor. Oh well, we'll see how hot the weather gets tomorrow!

A short across the sensor would give maximum lean correction.

In a modern car if one pulls the sensor connector the controller instantly substitutes a mapped value. As you saw those old controllers believe anything you tell them. At MB training class 1974 I was taught that the temp correction was about 30-35% of the total fuel calculation. That is if I remember correctly.

Progress! The car no longer seems to stall after a hot run. Plugs still look pretty clean, so I don't think I have a badly rich condition. Is this resistor a band-aid, or does it mean that the ECU/MAP adjustments are off? Is this a good long-term solution, or should I be changing the sensor (or something else)?

The idle on this car always seems to be a little lumpy. Is this normal for the D-Jet system?

Thanks, Steve for helping to diagnose this one! This car has had an eventful last couple of months, but I think that might be the subject of another topic.

Also was just wondering, is there a reasonable way to set the ECU/MAP settings without the exhaust analyzer? I've in the past changed the ECU setting when it seemed way off by idle/vacuum and by looking at the plugs.

Actually, I'm also wondering if the lumpy idle is possibly a result of incorrect valve clearances. How do the valve clearances influence idle, anyway?

Thanks for educating me. This forum is really great!

The sensor is a reasonable long term solution. Setting your mixture by long term performance (plug condition, fuel consumption), might actually work if adjustments were quantifiable. The clicks on the idle mixture compensator might be such, but MAP sensor adjustments are way too sensitive to be approached that way.

It should be done properly at least once to establish a base line.

This engine does have some inherent idle vibration (more so than later v8s). I haven't seen a propensity to poor idling due to valve lash on these but the valves should be properly adjusted. Having valves too tight allows some leakage of compression and disturbance of volumetric air flow.

Steve, did you mean that the resistor is an acceptable long-term solution? In other words, you don't think that I need to replace the sensor?

BTW, I posted a new thread about how the mechanic that replaced the timing chain and gears in this car only replaced one (broken) chain rail, the inner left one, I believe. Is this now a time bomb waiting for one of the other rails to break?

thanks again!