Not Your Everyday Idle!

[wild01ride]

OK, so recently my '98 has developed a hella crappy idle. Basically, the idle is REALLY erratic between 500-1000RPM. It's so bad the car shakes like a ***** when idling. The problem is also noticeable at speed with very low throttle position. You can literally feel the car surging while cruising at a constant speed while applying small amounts of throttle.

I've gone through the high idle thing before and conquered it, so I'm familiar with that. Naturally I thought I was having an IAC issue. I took the IAC off, cleaned it and put it back together- still no good!

OK, so I know everyone's already half done typing a repsonse about coils, yada, yada, yada... but wait!
At any higher throttle position, the thing runs like a ***** ape- as good as it has ever run. I mean, it's STRONG- and absolutely smooth as butter too, no surging, no hesitation, NOTHING.

So, I don't know if it could still be a faulty IAC or not. I wouldn't imagine that the IAC would cause problems at partial throttle settings, but maybe so. The other jacked up thing is that sometimes it'll idle without any problem. WTF?!?!

Oh, and then there's some kind of burning smell going on- smells like burning rubber (PS belt??), but smells like it's coming from between the engine and firewall. It gets pretty bad when stopped for a long time.

Any thoughts would be greatly appreciated!

 

[NiNeTy Fo SHO]

My dad had a problem just like this on his truck (F-150)...shaky when under light acceleration and the rpms were low. Also sometimes shaky at idle, but when on the gas and the rpms were up, it ran great.

His problem ended up being that one of his spark plugs were cracked. How this happens, I have no clue (heat I would guess?). He replaced his plugs and noticed that one of them had a small crack in it...and after that no problems with being shaky and idling.

 

[Mr Anonymous]

It is very common for a failing coil to misfire at idle but smooth out off idle. It could still be missing off idle, but the V8 SHO runs remarkably smoothly on 7 cyls above about 1500 rpms.

 

[SHOZ123]

I just had one fail and it will get worse enough to be quite noticable and finally will set a code.

 

[Ian Macoomb]

With a coil going south it is common to feel normal above 1500 rpm like others have said. It will seem like you have all the power you used to but if you time it , it will be slower. Also, with an open exhaust you will be able to hear a difference in sound.

 

[SHOCRUZR]

I concur

Mr. Anonymous, SHOZ123, & Ian Macoomb!

I had that issue last month. I was hoping that it can wait around for Spring then it got worse. I tried to disconnect the 4 coils infront one by one to see if I will notice a change on the idle. I couldn't really pinpoint or notice any difference between those so I bought 4 new coils and plugs. Changed the back coils and plugs with the new ones. The idle remained rough. Swapping the coils one by one and found out that the culprit is coil #8. All the boots of the coils are good no cracks, but the spring on coil #8 rattles like a busted bulb. All 8 coils have tiny cracks on the bottom close to the rubber boot but it doesn't seem to be a problem.

BTW, I cleaned my IAC valve, TPS, & MAF sensors with a Brake Cleaner before going for the coils.

I tried to test it using a multimeter but I don't have numbers to go by. I used SHOZ123's info at the v8sho website.