Wastes gas runs weird

[Tims450SL]

My 75 450 SL wastes gas and has a weird miss in it. I have new plugs (correct ones) fuel filter and fuel pump just put on. It does run screwy like it is fouled or something when started from cold 5 minutes it clears up. There is no cold starter high idle either. Any suggestions please???

[Roncallo]

I'm not sure if thats a Djet or Kjet. But in any case If you could get someone to put a scope on it preferably when cold that would be a bid help. If its a Djet a scope on the trigger points would also help, except you probably cant find anyone who knows how.

John Roncallo

[Richard Wooldridge]

When it "clears up", does it do it all at once, like throwing a switch, or does it gradually clear up? On the cold start fast idle, perhaps the cold start idle unit's heater went bad in the past, and someone blocked the unit so it would idle normally? I believe the cold start enrichment uses an additional injector in the intake air path, which is switched off as the car reaches operating temperature. If it clears up all at once, it could be somewhat normal if the cold start high idle unit is indeed defective and has been blocked.
On these cars, the temperature sensor is VERY important, as it has more influence on the mixture than almost anything else except for the air sensor. You might want to consider replacing it on general principles.
As Roncallo said, the trigger points are a good place to look for trouble, as they are usually neglected due to their location.

[Tims450SL]

thanks you guys- seriously. It clears up like in 5-10 minutes like it got warm...I have never had a car act like it. Almost like a plug is fowled out..(They aren't I checked just replaced all and not a speck of oil anywhere) It makes sense what you say, as it clears up runs great slight miss at idle...the cold start idle is like it is not idling normal; it needs for me to give it gas and let it warm up alittle. after 1-2 miles it clears up-but in the mean time the gas is like going fast-I mean it you can see it move. Please explain what you suggest me replacing- the air or idle sensor? What is this think that could be blocked off? I have to have this fixed it is just crazy- but I appreciate your assistance very much! What are trigger points? Do you mean the points in the distributor?

[ScottinSoCal]

75 uses the D Jetronic system, an early electronic fuel injection.

You've got two temperature sensors in the car, one on the driver side close to the firewall and one in the front of the engine on the passenger side. Either of those could be bad (or both - they were both bad on my 75). You've also got the auxiliary air valve that controls the idle. It sounds like your car has a couple of things going on.

I found this thread invaluable for diagnosing my car:
http://www.benzworld.org/forums/w116-s-se-sel-sd-class/1223523-bosch-d-jetronic-k-jetronic-troubleshooting.html

I also found this thread valuable for fixing my auxiliary air valve:
http://www.peachparts.com/shopforum/sitemap/t-74837.html

There are a couple of links in there that take you to a Volve site, but Volvo used the same D Jetronic system used in your car. The idle problem is probably related to the AAV.

You've also got a cold injector designed to make the fuel system run really rich during very cold starts. It triggers off the temp sensors in your car, and that's why I suspect one or both yours might be bad. You can check the temp sensors with a standard volt/ohm meter. If the resistance values are outside spec limits, replace them. It's less than $200 for both. I had to buy one of them from the dealer, because I couldn't find it aftermarket, but the price still wasn't bad.

The AAV on these cars is all mechanical, but it works on the same principle as your thermostat, and that means it wears out. Unfortunately it's hellaciously expensive to replace and very difficult to track down. A popular fix when they die is to put a restrictor in the AAV air line to the engine. It slows down the idle, but it doesn't react to changing engine temperatures. People usually optimize the air flow for warm operation, which means cold running is less than optimal. With your engine it's very likely that what you're giving it more of when you press the gas pedal isn't fuel, it's air. That will make the engine run better at cold idle if it's starved for enough air to burn the fuel that's getting dumped into it.

[Tims450SL]

hey scott- thanks- I get it...I suspected that the cold start is bad as it runs like really bad at warm up- 10 minutes it runs great. So I am going to get those sensors and someone also mentioned that I will need a cold start switch/relay or what ever it is. I really appreciate your info as I am going to call my guy who by the way has everything you could need at great prices in ftlaud. his name is Mike at Overseas Parts. He actually can get anything in 24 hours or less. So I am going that route and I appreciate your help alot! Where in scal are you? I am from malibu and laguna.

[ScottinSoCal]

Originally Posted by Tims450SL

Where in scal are you? I am from malibu and laguna.

I drive through Malibu a lot - I live up in Ventura County. Any time I have to go to LA it's just easier to take PCH down and cut across instead of going down 101 and having to deal with the 405.

[Tims450SL]

exactly my route I have a place in Cross Creek. I have my 1960 300 SL there. It's the red one if you have seen me blastin up and down PCH.

[Tims450SL]

oh his number at overseas is 954.561.9300