CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

High idle and sticky throttle after spark plug change

I replaced the spark plugs in my 02 Nissan Quest, and it now idles at about 1000-1100rpm… until I drive it. Then it idles higher. If I drive it fast it idles much higher, such that if I take my foot off the brake (automatic) it will reach about 40 or 45mph without touching the accelerator. I’ve no idea how to proceed. I disconnected my battery in an attempt to “reset” something, and it did back my idle down to the original 1000-1100rpm, but did not correct the issue. I also did check to make sure the throttle cables are clear and not crimped or sticky. I used bosch platinum plugs which the computer at autozone indicated were ok for my make and model.

How many miles, road or city driving, and when were the plugs last replaced before this time? Why did you replace them, were there other problems before you replaced them? What was the condition of the old plugs. Were they all the same, or were some different?

The Bosch plugs should not have affected the idle. My guess is that you disturbed something in the process of changing the plugs, perhaps a vacuum hose, and that affected the idle. Go back over your work.

About 112k miles, mixed highway and city (small town, open roads). I have never replaced the plugs, vehicle had about 40k miles when I bought it. I replaced the plugs because my fuel economy has steadily declined and it seems to hesitate when I want it to go. Just doesn’t have the juice it used to. Also I had never changed them, and I figured they were due. The old plugs had some black carbon residue and some white residue closest to the spark point, but were otherwise pretty clean (a bit rusty where the socket grabs them though). The tips had a 45degree chisel shape at the ends that had worn off. All of them were identical or very nearly identical in condition.

Did you gap your spark plugs to the correct gap? It’s a common misconception that new spark plugs don’t need to be gapped, they do. If the gap is way off or touching the center electrode the computer may be taking over to keep the engine running (higher idle). I know it’s left field here but a possibility.

These have the 4 prongs so I’m not sure how to gap them, a regular gapping tool won’t fit. I did look at them as I was installing and none were “way” out such that I’d notice (like touching). Also I’ve heard before that gapping a platinum plug may damage it? Oh and going out to driveway to check all the vacuum hoses as well. I did change the air filter out at the same time and it was very difficult to get to, I might have pulled something out fussing with it!

from the additional information you give, I’d agree that the plugs are not the problem. Check the timing, and the spark, is it hot blue or cool yellow? Also check the vacume hoses for cracks and leaks, and check the fuel filter. I would also take it to a chain parts store, and have it tested for codes, even though you do not have a check engine light. It could be something in the automatic fuel regulation system.

I’m afraid checking timing and spark exceed my limited mechanical abilities. I did look over the hoses and couldn’t find anything (well, I found one that was “disconnected” but it traced back to the tranny, I think it’s a breather hose since it’s curled at the end and very dirty like nothings been hooked to it in a long time). The fuel filter was changed at my last oil change in an initial attempt to fix the sluggishness. Sounds like time for a mechanic. I can drive it safely as long as I don’t get on it and stick the throttle up too fast. I will say this, it doesn’t hesitate anymore! Just have to have a heavy foot on the brake and be prepared to shift to neutral if it doesn’t want to stop! Thanks for all the advice!

Four ground electrodes? That would be the “Platinum +4” plugs with the Yttrium-enhanced electrodes?

While I can’t see how these could cause higher idle, these are not original equipment replacement plugs, and you got took. Sorry.

I’m curious. Try replacing them with proper OEM replacement plugs and see if the idle calms down. Try NGK’s. Post back with the results.

I starred your comment because I have been recommended oems from the parts guy, though it was because cars ran worse than if they put the old plugs back in, not better, but who knows, worth a shot in my book.

I agree mountainbike. I have seen engines that changed idle after a botched spark plug change. It was the computer trying to rectify the situation. I don’t like those multiple electrode plugs at all because they fall into the category of “gimmicky”.

Yup the +4 was on the box. I’ve got to take it to the store to check for codes anyway, I’ll grab another set.

Didn’t make it to the store tonight, but I did have time to swap the old plugs back in just to try and isolate the issue. With the old plugs it still idles high. If this helps… when in neutral if I rev the engine up to about 4k and take my foot off the gas, the tach “steps” down about 1k at a time instead of rolling back down smoothly. When in drive it performed similarly, although the steps weren’t as sudden. I looked harder for a loose vacuum line and could not find one.

I’m going to take a wild guess and guess that you knocked off the electrical connector for the throttle position sensor. This sensor is turned by the throttle, so look for it where near where the throttle cable connects to the accelerator linkage under the hood. Another possibility is that you knocked off a vacuum line from something and the extra air is making it idle high.

If it’s the throttle position sensor, the check engine light should have turned on. A vacuum line might not turn on the light.