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2002 Neon revs up in park after a visit to the mechanic

Took my car in to get the tire and headlight replaced. Not my usual mechanic. While they were in there, they took it upon themselves to open up the filter and root around the throttle body. They said it was heavily gunked up, and asked to clean it. The car has been having serious starting issues, so this made sense to me, so I overlooked them examining a part of the car that had nothing to do with what I’d asked them to repair, and told them to go ahead.

Mechanic cleaned out the throttle body. Now he says there are scratches in it, he suspects from a mouse or rodent. Says it needs to be replaced, but he couldn’t source a part today. Either way, I can’t afford that. Says it’s fine to drive, though.

Whatever they did, the car is now worse. Engine revs up high in park or when I start the car, and then drops dow and keeps revving rhythmically while idle. And I swear that while in drive, it’s running faster even with my foot off the gas, and not dropping out of higher gears as fast. I go back and confront the mechanic. He says yes, that’s all because of those scratches. But I don’t understand how before, when it still had these scratches, WASN’T clean, when it was, in his words, all gunked up, it was working BETTER than it is now, when it’s clean and just has those scratches?

It’s tough to guess until the mechanic completes his repair. Not all parts are metal, and rodents do constantly chew on everything in order to keep their teeth from overgrowing. If some small rodent did manage to get in there, it is possible that the critter did some damage.

See how it runs when the mechanic has completed his work. Post with the results.

That IS how it runs now that he completed his work. Prior to him cleaning out the throttle body, the car had trouble starting in cold weather, but otherwise ran (and more importantly for this, idled) normally. Now, post-cleaning, it revs up very high at ignition or when I throw it into park, then drops back down to a still not-idle level, revving rhythmically.

"Says it needs to be replaced, but he couldn’t source a part today. "

Did I misunderstand these comments? If not, than his work isn’t finished.

I think what moutainbike meant about completing repairs is that you implied that you still needed to have the new throttle body installed. So clarify. Did the new throttle body go on or not?

If it was the driver’s side headlight, then the air box would have come out in order to get to it. So pulling the air box + headlight can very well be related.

In any case, it just sounds to me like things didn’t get buttoned back up all the way, and it left you with a vacuum leak. This is pretty easy to do by just missing something small. I’m sure they can easily take care of it. Or if you know your way around under the hood, check that the evap line got hooked back up to the throttle body, that the vacuum lines went back onto the air box, that the IAC valve wasn’t left loose on the TB, and that the worm clamps for the main intake hose are tight.

Of course, if the TB is scratched up that badly, then it could be a problem and wasn’t before because the gunk actually helped seal up the throttle plate. I do find this to be odd. How long have you owned the car? The only way for a rodent to get to the TB would be to go through the airbox and that would mean eating through the air filter. It would be hard to miss. Or they could chew through the intake hose - and that would be impossible to miss.

Apologies. He said they’d cleaned out the throttle body, and screwed everything back in, but that there were scratches on it, and I should get a new one. He couldn’t source a part today, but they could find a part sometime this week and quote me a price. In the meantime, it was “fine to drive”.

It was the passenger side light. Nowhere near the air box. I left there in a pretty big huff after he stonewalled me about the idling. I’m not sure whether coming back and telling him to do more is in the cards, especially if he’s going to bill me for it…

I was thinking more of possible damage to the IAC orifice (or motor) or perhaps even the TPS, both of which are part of the throttle body assembly. Without knowing exactly whet he was looking at, it was only a guess.

But I like Cig’s comments about a possible vacuum leak even better.

I have an '01 Neon and have had the TB off & airbox out several times. It’s pretty easy to forget one or more of the connections. The PCV line also feeds into the intake right there and though it doesn’t have to come off, it is easily disturbed or even just removed for easier access.

@Mordecai, if you want a reasonably systematic list with locations then let me know and I’ll pop the hood to get specific. If you don’t plan to go under the hood yourself, then obviously it won’t do any good.

cig, I’d really appreciate that, but I have to admit, while I really would like to take a look at it myself and and fix it, if it were just a case of tightening some screw, the fact is I wouldn’t even know what I was looking at.

New wrinkles: Had to go on an errand I couldn’t delay any longer. The car drives as if my foot were always lightly on the pedal. Even at highways speeds, if my foot is off the pedal, it is NOT decelerating. Also, when I brake to a stop, the car suddenly, and momentarily, tries to accelerate. The car never drops below 1000 RPM, even at rest.

Yep, a vacuum leak is a definite possibility. You have the classic symptoms.

The problem might be with the Idle Air Control valve.

This is mounted on the throttle body.

With the engine idling, take the handle of a screwdriver and rap on the IAC valve. If the engine idle speed changes when doing this replace the IAC valve.


Just gave it a try, Tester. No difference.

It is possible the idle air control motor was failing and the cleaning solvent finished it off.

The hose connecting the throttle body to the manifold should be inspected to see if it is properly installed.

Your tire mechanic did something he should not have, cleaning the throttle body.

2000-2005 2.0L Neons have plastic throttle bodies and solvents should not be used to clean them.
Harsh chemicals can strip the protective coating from the plastic surfaces inside the throttle body.

Does the accelerator pedal feel sticky now? The throttle plate may not be closing properly.

Accelerating is fine. It’s slowing down/stopping that’s the problem now. Due to the constant low acceleration, I definitely have to exert more force to brake, and as I said, when I come to a stop it suddenly revs up for a moment before semi-relaxing. I don’t actually feel safe driving it now, but I have doctor’s appointments I NEED to drive to in the next 2 days…

Well, my son drove off in the Neon and I fell asleep on the couch…

Neon throttle bodies are plastic but can be cleaned, and frequently do need it. It just depends on how they did it. Typical auto cleaning solvents are not recommended.

Anyway…you’ve apparently found the IAC so double check that it isn’t loose. Down under it there should be a rubber black elbow attached to a piece of small black plastic tubing. It pushes onto a port on the engine side of the throttle body. Make sure that is on there. If not it will just be a little connector stub and the hose that connects to it should just be floating around down there somewhere.

The throttle body connects straight to the air cleaner box. Wiggle on the throttle body to make sure its tight against the box. The entire assembly will wiggle some - but the air box and TB should wiggle together. Then between the throttle body and engine is the main intake duct - the big black accordion looking tube. On either end is a big silver hose clamp that is tightened and loosened with a screw on it. The screw head is probably 8mm but will also take a flat blade screwdriver. On the throttle body side the screw maybe hidden under that black elbow (above). Make sure those clamps are tight.

On the engine side, the intake duct connects to the intake manifold. And right next to where it connects a hose comes in from above and attaches to a stub port on the intake. That is your PCV line. If you follow it upwards it should attach with another black rubber elbow to the PCV valve which is screwed into the side of the valve cover. Look at that entire piece of tubing, including where it connects to the intake. If anything is amiss there (split or crack in that tube, not connected at the intake) it would account for your symptoms.

Think this way - that whole area is main air intake for the engine. The only way air should get in is through the throttle body. The engine is also desperately trying to suck air in. So look for anything like that. Of course, you might just need a new throttle body too, since if that was damaged (whether by rodents or the shop) then it’s not doing its job right and is letting too much air by.

Alright, with the help of a friend, I looked it over. There appears to be a big air leak from the main intake duct. We were in a parking lot, so there wasn’t really space to pull it apart and see if it’s cracked, but that’s definitely a place where the vacuum seal is breaking. Putting a hand over the throttle body does nothing. Before we arrived at that, we unplugged the IAC, with absolutely no change.

We opened up the air box and looked at the throttle body itself. The throttle plate looks like it’s forming a seal fine but now I see the scratches the mechanic was talking about. Except it doesn’t LOOK like scratches to me. It looks almost…blistered? LIke the center of the plate melted slightly, even though the outside looks fine. And also, the inside of the air filter, replaced 3 days ago, looks scorched. Not just full of gunk, but actually scorched. Outside looks fairly clean.