: 99SLS Ruff idle when started

06-30-05, 01:15 PM
I have a 99 SLS 50,000K It idles ruff when started. It is getting worse in the past few months. When I took it in they couldn't find anything wrong with it. After driving for a few minutes seems to go away. Gas milage does not seem to be effected. Any ideas what it might be? Thanks in advance.

06-30-05, 06:20 PM
That is the first symptom I noticed on my 97 STS, which turned out to be a leaking head gasket. Keep an eye on coolant consumption. If you are not using any coolant, there may be a fuel injector that is not seating completely after the engine is turned off. (There is still some fuel pressure and it leaks past the injector seat, and floods the cylinder so it runs rough for a short time on the next start.) I have seen this happen on other cars, but I do not know the fuel system on the N*, so I could be wrong about this.

06-30-05, 06:50 PM
It is possible it's a head gasket, though doubtful with those miles on it. You can have a leakdown test done, cylinder pressure test, or hydrocarbon test done to help see if it's a HG.

Could also be an injector like air1ren said, but I'm guessing that once the engine gets to temp it goes away?

I had the same problem, but I had the hydrocarbon test done, it detected some but not enough for a HG, I then had they cylinders tested, again all good (150+ psi in all cylinders) and then finally a leakdown, all negative. The source of my problem was finally diagnosed by chance when I replaced a radiator as a leaking intake manifold gasket. I checked it out using a bottle sprayer with water in it, i just let the engine sit for a day, then started it, she was bucking and skipping a little, and then i sprayed water around the edges of the intake manifold and sure enough, it started missing when i got to the back bank, rpms dropped, it sounded like crap. This had to be done when the block was cold cause once it heats up everything expands and seals the leak. It's definatley not good for your engine but won't kill it. I'm actually going to be doing this repair this weekend and documenting it in my other post in this section: http://www.cadillacforums.com/forums/showthread.php?t=44587

06-30-05, 07:32 PM
thanks for the help. I actually own a 95 and a 99 SLS and I like the cars but to be honest I am finding some of the same things going wrong with both of them. I just posted one about my 95, As stanger as it sounds both cars had to liquid leak out of the drivers side mirror.

07-01-05, 12:57 AM
my 93 sts was doing that, check the plug gaps. they should be .5... but check the book.

07-01-05, 10:55 AM
The correct spark plug gap is 0.050".

07-01-05, 09:41 PM
Change your wires. You may want to change your plugs while you're at it. I had the same prob with my 98 eldorado(60,000 miles) and I wasted money buying a fpr.I changed the plugs & wires and it starts/idles fine ever since.

07-01-05, 09:55 PM
I'd be inclined to think plugs and wires as well though it does seem premature at that milage.

07-01-05, 10:13 PM
Too many assumptions here. Who is "they" that didn't find anything wrong? Did "they" pull codes?
"not finding anything wrong" has always been a pet peeve. It implies the driver is imagining things. Obviously something is wrong.

I think it's a sensor somewhere. Lot's of things change as the car warms up. Fuel enrichment for one.