This is a recurring problem I have had ever since I bought the car. Once the car is warmed up I'll stop for 40 minutes or so. When I start up again it misfires. Codes show multiple misfires all eight cylinders. This goes for 20 seconds or so and then it smooths out and runs fine. The check engine light comes on but it doesn't go into restricted performance mode. Tried BG throttle body cleaning, SeaFoam, new plugs, new fuel filter. No codes except the misfire. I'm stumped, my mechanic is stumped. Anybody else had this happen. BTW, this only happens when the engine is warmed up. Cold starts are fine.

 

Could it be a temp sensor sending results that are within it's scope, but still wrong? On my Maser I've had water temp sensors beeing correct on 95% of it's scope, the other 5% it gives low voltage or high voltage, messing with the mixture.

Cheers, Jarle

 

I have no idea why I came up with this but have you checked your 02 sensors? I do not have any background to justify my suggestion other that reading a similar situation. Let us know what you find.

 

I'll pass the suggestions of 02 sensor and temp sensor to my mechanic on Monday.

 

What did you find?

 

Mechanic doesn't think it's O2 sensors since it only happens at start when engine is warm and no miss detected after the first few seconds. He thinks an injector is leaking fuel into cylinder after car is shut off. Then if the car is started again before fuel dissipates, I get the multiple misfires. He said to try another can of SeaFoam and see if that makes a difference.

 

Are your heat shields still on over and around the cats? Especially the drivers side were the fuel lines come up from under the car and around them. If not or even if they are, you may be experiancing good old fashion heat soak of the fuel in the lines and partial vaporization.
Also try to find fuel without ethanol, i know its almost impossible around large cities, but we see alot of ethanol induced starting issues

 

Brutal,
Never heard of the heat soak thing. Is there a cure? As for ethanol, I haven't seen non ethanol fuel anywhere in Florida. I use either Mobil or Shell and both have ethanol in the mix.

 

Its minimized with higher fuel pressure used in fuel injection systems. But when the fuel sits and it over hi temp items like the cats, it can cause partial vaporization which can lead to stumbling when the vapor hits the injectors at startup. There are alot of insulation materials and wraps to allieviate this issue. I know ethanol is a way of life nowadays, but we see issues with it and many times when tested can find more than 10% in the tank. We are also finding whitish flakes or deposits on the injector tips from ethanol. This may have nothing to do with your issue, but a possibilty

 

Brutal,
Hey, is there a way to confirm that's the problem? And is there a solution?

 

The solution is to cover and wrap fuel lines on the side of the engine compartment where they come up past the cat converter and under the coolant reservoir. How often does it happen?

 

It is an often occurrence. I'd say 60% of the time when starting the car after it has been sitting for 30-60 minutes.

 

Does this take place in warmer climates?

 

I'm in Florida so warmer climate is all we have.

 

I know that the old Hemis had a problem with heat. We would pack the fuel line with ice at the drag strip. We took a coffee can and coiled the fuel line in it and off we went. Won a few trophies that way. Yes I know it does not apply!

 

the other issue you may have is the fuel pressure bleeding down form a leaking injector. on a cold start the fuel has evaporated from the previous trip. cold start also need rich mix so less of an issue. on a hot start the vaporized and wet fuel leakage into the ports/cylinder will make it harder to start. 1) for the rich mix and 2) lack of fuel rail pressure.
you can check by hooking up a fuel pressure gauge to the fuel rail and watch what happens after you drive and then stop and let it sit. Tape it to the windshield and drive around withit and see what happens. if it bleeds down the only way to know which 1 is to remove the injectors and send them out to be tested and cleaned. If you have 1 or more that cannot be repaired and cleaned to work properly then you need to replace those.
I would recommend injectorrx.com here in Houston
http://injectorrx.com/

 

I think you have an induction system leak, you are leaning the mixture out, but its fine when cold, since the injectors are running rich. Look for an air leak between the MAF and the throttle plate.

 

I reread your post and forgot the car ran normal after the misfiring, forget the air leak. It is probably fuel vaporization although I have rarely heard of it on these cars, or any fuel injected car for that matter, since the fuel return system keeps the fuel cool. The 4.2 cars do not have a fuel return system so I guess they use a different method of preventing vaporization, probably the high pressure of their system (70 psi instead of 42 psi in the 4.0).

 

But this should show as a lean engine condition since it is air going in the engine the mass air flow sensor is not reading. Like when the air intake tube bellows deteriorate and start cracking apart you get p0171, p0174

 

Finally solved the problem! I had it in for a fuel smell and my mechanic replaced the purge valve. The rough idle/misfire at start up has not happened since! Two birds with one stone.