Warm start rough idle 72 4.5
 

I have a 72 280 SE 4.5 that runs very rough like it is going to die when it is re-started and already warm. It starts fine when it is cold but once warm and shut off for 10 minutes or so then re-started it has a rough idle. After 30 seconds or so it runs great. Any suggestions? :o

You might try freeing up the Aux Air Valve. They tend to stick which is same as having a vacuum leak because it bypasses air into the intake manifold.

Try going for a good run and then adjust your idle speed with car hot and see if that makes any difference. Your idle speed may be high when cold, but may be better when hot.

The other thing I have found here in Canada, is that they add butane and other winterizers to the gas at this time of the year. That along with the ethanol already in the fuel reduces the boiling point of the gas.I think the gas vaporizes in the injector nozzles causing hot-start problems. I increased my fuel pressure to about 30-31 psig and it seemed to help a lot. (If you have manual, it tells you how to check and adjust fuel pressure)

Good Luck

Thanks
 

:D I am still learning and am waiting on my shop manuel so I will check it out once the book arrives. Thanks

If, as you say, it's fine on cold start (when it's in fuel enrichment mode) and it's fine after about 30seconds when hot; you likely have a bad fuel injector spray pattern. It's not noticed when cold because you're in enrichment mode, and fast idle mode. But when hot, having injectors that are pissing streams instead of a rotating fog pattern, will cause your symptoms in a CIS injected car.

The easiest route is going to be to dump a can of BG44K (get it on ebay) into the fuel tank, on 1/2 tank of fuel, and then go for an hour drive on the freeway (30min out and 30min back). By the time you get done with that 1/2 tank of gas, you should notice a change. If you don't, consider removing the injectors and cleaning them yourself or having them cleaned.

However, this is also a good time to be checking for vacuum leaks.......around the injectors, at hose connections on the intake, and from the Auxilary Air Valve (as previously suggested). Although I doubt these are your problem because you're saying it runs fine after 30sec. Still, any vacuum leaks will contribute to poor hot idle, even if you barely notice because it's a V-8 and you have so many pistons making power.

So dump in a can of injection cleaner, run up the hill to P-Town under load, and by the time you get home, it should be much improved.

Always a great help
 

Todd,
You always have great advice. We are going to do this right away!:)

A minor correction, this is not a CIS engine. This is D-jet, which is electronic.

I would also have a look at the cold start valve. It's supposed to fire only when the engine is cranked when cold, but if the relay is bad, it may be firing during a hot start, causing the engine to run rough until it burns off the extra fuel.

Thanks. I ran a can of BG 44K and it seems to be running like a champ but I may want to replace the cold start valve as well. Where is this valve located?

Sorry Chuck, I was thinking these were CIS with the flapper door there in the middle of the valley. Good point about the cold start leaking. I've seen injectors that were dirty and dripped after the car was shut off too. Then the carbon on the backs of the valves and the tops of the pistons soaks up the fuel. When you go to start, you're temporarily flooded while all the extra fuel burns off.

Glad the BG helped. It's the best fuel injection cleaner on the market. Use it every few months, and you might hold off on replacing anything until you run a tank of fuel through it. Then see how it is.

The CSV is right at the front of the intake manifold. You can pull the elecrical connecter and try a warm start. If it doesn't miss, you've found the problem.

That's something to add to my to-do list. I think I might also put a meter on the disconnected CSV connector and see if it does activate when engine is hot.

BTW - Merry Christmas to all on this great forum!

Graham
'72 350SL 4.5