Misses in the morning

[bill4247]

I have a 280SEL 4.5. In the morning in starts instantly and runs smoothly for about 5 miles then suddenly begins to run very rough for about 5 miles and then equally suddenly smooths out for the rest of the day. This happens more of less every day until recently when it has begun running rough for a longer period. My only idea is that the cold start injector gets stuck open? or the sensor is bad? Doesn't seem to smoke excessively while running rough. Could it be an ignition problem? Generally it runs rich probably because of vaacum leaks. Any ideas about where to start looking would be greatly appreciated.

[kpb]

bill, i've got a similar issue with my 4.5. by happenstance recently popped the hood right after startup and found leaking f.i hoses. I replaced all the pieces and injector seals last year but hadn't gotten around to the rest of the rubber on the fuel rail (until yesterday). Watching it for a few days, I noticed the leaks only appeared on cold starts, not when warm, so I thought maybe the cold start valve added extra psi to the fuel delivery system?

Anyhow, since renewing the fuel rail rubber it has smoothed out quite a bit but I still experience a pretty lumpy idle when warm. My vacuum to the door locks is disconnected BTW so there should not be any leaks hidden from view in the vacuum system.

[bill4247]

Thanks, very interesting. I'll pull over next time it happens, pop the hood and take a look. Does feel like fuel starvation and there is a lot of old rubber in there.

[Earlybird]

You may be experiencing the old crank case oil on the EFI points problem. They reside under the distributor breaker plate.

[Tomguy]

FWIW: It doesnt sound like oil in the tirgger points, oil in the trigger points causes rough running cold until it's warmed up. It won't start out smooth, get rough, and then smooth out again - it'll be rough until it warms.

Sounds like an issue with perhaps your cold start valve staying on (way) too long. If it happens again, directly unplug the cold start valve and see if it smooths out. If so, you need a new thermo-time switch. It sounds like something temp-related at any rate. Could be the water temp sensor or the aux air valve too. if you want to test the water temp sensor, readings from it should be close to these (figure a 10% difference is fine, if it's WAY off, you need a new one - it says these are "heavily temperature dependent"):

5.9 kohm @ 0° C
2.5 kohm @ 20° C
1.2 kohm @ 40° C
600 ohm @ 60° C
325 ohm @ 80° C
190 ohm @ 100° C

I'd imagine the best way to test these would be pulling the sensor and putting it ice water (It should be very close to 0°), look for anywhere from 4-7 ohms since the water probably wont be COLDER than 0°. 20° C is 68°F, close to room temperature. 40°C is 104°F. These 3 temperatures/ranges are the most crucial during warmup.

[bill4247]

Thanks Tom. Will do as you and others suggested. Just one question. Is it possible for the cold start injector to shut off and the aux air valve to stay open for some time causing a temporary lean mixture? Cause I don't see any smoke like it's running too rich when it misses. Or do the cold start injector and aux air valve always work together?

[Tomguy]

The aux air valve is controlled by water temperature (no electronic input or controls whatsoever).

The cold start valve/injector is operated off a thermo/time switch in the coolant. It's got a bimetallic spring in it and a small length of wire that heats said strip up. Whenever electricity is passing through it, it's heating the strip. Once it hits a certian temp, it switches off and the cold start injector turns off.

Completely unrelated systems, but either one could be causing the problem (or both in conjection) - if you're sure it's from too lean running and not too rich then it's probably not the injector. However, pulling the plug on it is a very simple test, and you could probably tell by the engine's idle speed alone if it made a difference or not.