fragmare

So I've been trying to track down a pesky idle problem in my 93 DX (D15B7) since last summer. I've already checked for vacuum leaks, cleaned the IACV, cleaned the FITV and throttle body. All to no effect. In fact, it's gotten worse over the winter. The idle will spike to about 2000 RPM on a cold start, then roll between 1500-2000 for about 30 seconds, then resume going back down normally. The idle will also sometimes briefly drop to 500 or below when stopped at a light or stop sign (which in turn causes the A/C to kick on/off rapidly for a couple seconds).

Recently, I noticed my engine burning more oil than I'm comfortable with and suspected bad piston rings. I did a head gasket job last summer and believe I accidentally got some of the old gasket material into the cylinders, which is rough on the rings (i know, i know. i should have been more careful scraping off the old gasket remnants.) Anyway, I went ahead and did a full leak down test on all 4 cylinders and came up with some rather bad numbers. Here are the results:

Cylinder 1 - 40% leakage
Cylinder 2 - 55% leakage!
Cylinder 3 - 40% leakage
Cylinder 4 - 25% leakage

The only place air was escaping was the dipstick hole, so piston rings are indeed the likely culprit.

My question is this... if the piston rings are worn enough, will that disturb the engine vacuum enough to cause low and/or rolling idle problems?

SiWannabe

I'm not sure how you came up with your %s, but if your leakdown/compression is as low as that makes it look then I think at this point, the idle question is kind of a moot point. You need rings. Throw in main and rod bearings while you're in there. If there was material in the cylinder, check them. If you don't know how, or have tools to, take it to a machine shop. Don't just slap it back together to find that you have an out-of-round hole.

To answer your question on bad rings having an effect on idle? The dropping to 500.... I would say yes. The other behaviors? ehhhhhh, just do the rebuild and during it check EVERYTHING 3 or 4 times. Have a friend look over your hose/electrical connections to make sure you didn't miss anything. Inspect hoses for leaks/wear.

Good luck.

fragmare

I did the leakdown test by calibrating the tester to 0PSI and then attaching the tester to each cylinder while at TDC and in their respective firing order (1, 4, 3, 2).

As far as replacing the rings, I plan on doing that later this summer. I'm doing the rings, crank bearings, rod bearings, front/rear main seals, oil pump gasket, oil pan gasket, new head gasket, valve stem seals, honing the cylinders... basically an engine rebuild.

I'm just wondering if the idle problem is tied in with the cylinder leakage... i started putting 2 and 2 together last night after i tested the cylinders and remembered the idle problem didn't start happening until AFTER i changed my head gasket last summer. Right about the same time the car started burning excessive oil.

SiWannabe

If you've checked absolutely everything else.... I honestly don't think rings could case irratic idle. Low enough compression to cause a 500RPM idle would probably be misfire induced and lead to a CEL.... Try the intake manifold gasket. To test it, spray a NON FLAMMABLE aerosol product (unless you wanna catch your car on fire) around the mating area while the idle is irradic. If it smooths out when you hit a spot, you have a leak. Honestly, if you plan to do all that work soon, I would probably just wait and maybe have the IM surfaced before the new gasket. Lol, or you could do like a buddy of mine did and put a bead of Silicone around it. :-D No but seriously, don't do that last part.

fragmare

Yea, I've exhausted nearly every other option. The ONLY thing I have not yet tried is replacing the FITV outright or screwing the white plastic piston inside the FITV all the way into its bore. When I cleaned and reassembled the FITV, i just put the plastic piston back in its original position. I've done plenty of poking around the various vacuum hoses and intake manifold with starting fluid while listening for a change in RPM... nada. I've adjusted the idle so it never drops to 500, but having the A/C on still makes it drop to 500 (but only briefly, when coming to a full stop). I'm theorizing that the low idle is due to the overall wear on the rings, where as the erratic, rolling idle at cold start might be due to the wild variation in compression between the cylinders. With one cylinder at 25% and another one at 55% (closer to 60, actually), the power balance of the engine is wayyyyy off.