What the heck?

[Arkaign]

Test driving a car, Black 2000 Honda Prelude SH, 5 speed, 81k miles. Guy wants $5,500 for it, and it's in pristine condition, aside from some drivers seat wear. Engine runs super smooth, and it shifts, steers, brakes perfectly. But there is one weird problem :

When I come to a light, or just park with the engine running for a moment, instead of idling at 800-1000 rpm, the engine rpm bounces up and down between 1200-2500 or so, sounds like I'm revving the engine at somebody (got a dirty look from a cop that was next to me at a light, lol!).

What would cause this, and is it an easy fix? Something in the fuel delivery system? Or the computer maybe? There are no mods on this car, it's bone stock. When you drive down the road, the throttle is dead even, or if you stick it in neutral and give it a given throttle level, it remains even, it's only when you have the clutch in / neutral, with your foot off the throttle that it does this.

Ideas? Thanks in advance.

 

[ViviTheMage]

weird that its between 1200-2500 ... otherwise I would have said an odd misfire in bank 1....could be a fuel injector (WEIRD, but it happens)....but you are right, most likely something with the fuel system.

Do you feel a loss of power when accelerating...are there any oil leaks around gaskets that you can see in the engine?

 

[SVT Cobra]

Sounds like a dirty mass air flow filter, which can cause cars to idle rough. But, it could be other things (as that sounds really really rough). It could also be o2 sensors, or a number of other things. Don't buy it, as it shows the condition in which it has been kept.

 

[ViviTheMage]

Originally posted by: SVT Cobra
Sounds like a dirty mass air flow filter, which can cause cars to idle rough. But, it could be other things (as that sounds really really rough). It could also be o2 sensors, or a number of other things. Don't buy it, as it shows the condition in which it has been kept.

exactly...unless the owner fixes it before he sells it to you.

 

[BD2003]

Sounds like a MAF or IAC (my best guess) problem to me...could be an easy fix. A dirty mass air flow sensor might sense less air flowing through than actually is flowing, and then the idle air control system opens the throttle even wider to compensate. Although it would have to be REALLY, REALLY filthy or just plain broken to cause it to rise 1000+ RPM though, so it could be entirely something else. Although the fact that it only happens at idle and not when you manually apply any throttle (standing or moving) really leans me towards IAC.

A honda specific board might have more explicit answers though. Is the CEL light on? And is/has the car been modded?

 

[Zenmervolt]

Sounds to me like the idle air control valve. Simple fix. Sometimes they just gum up a little.

ZV

 

[BD2003]

Originally posted by: SVT Cobra
Sounds like a dirty mass air flow filter, which can cause cars to idle rough. But, it could be other things (as that sounds really really rough). It could also be o2 sensors, or a number of other things. Don't buy it, as it shows the condition in which it has been kept.

Yeah, its really not a good sign. Although seriously, check the honda boards. I got a killer deal ($2500-3000 below KBB) on my car because of a dirty MAF sensor.

 

[Arkaign]

You guys are awesome! Best forum ever The guy selling it has already bought a Nissan Titan, so he's going to give me over the weekend to check the car over, and even tossed me an Autozone gift card with about $60 on it for any parts/misc to put towards the car, as long as I bring him the receipt. I'll see what I can get done and post back here with the results. I really want this thing to work out, it's a beautiful car, I had a 5th gen before, but it was wrecked. Too bad I can't afford another Jackson supercharger

 

[jtvang125]

Aftermarket cams?

 

[Arkaign]

The car is stock, everything as it came from the dealer AFAIK and from what he's told me. I've got a mechanic friend driving the car now, he's going to take it to his garage and check out the air/fuel sensors and filters.

 

[exdeath]

In addition to MAF and IAC as suggested already... could also be a vacuum leak. Those suck to find.

 

[Colt45]

my money is on idle stabilizer valve and/or control circuitry

 

[ayabe]

Originally posted by: Arkaign
You guys are awesome! Best forum ever The guy selling it has already bought a Nissan Titan, so he's going to give me over the weekend to check the car over, and even tossed me an Autozone gift card with about $60 on it for any parts/misc to put towards the car, as long as I bring him the receipt. I'll see what I can get done and post back here with the results. I really want this thing to work out, it's a beautiful car, I had a 5th gen before, but it was wrecked. Too bad I can't afford another Jackson supercharger

Hehe, I thought a couple days ago you were saying how the car was too rough on you for the long commute. Or am I getting you confused with someone else?

 

[iamwiz82]

Originally posted by: exdeath
In addition to MAF and IAC as suggested already... could also be a vacuum leak. Those suck to find.

Having just dealt with this exact same problem a few weeks ago, this was my issue. I used a vacuum gauge to confirm and then used a long piece of tube to poke around the engine bay with my ear on the other end.

 

[myocardia]

That car needs a new throttle position sensor. None of the other things that have been mentioned so far can cause the engine to rev, with your foot off the throttle. Here's how to make sure that's the problem: try giving it ~1/2 throttle, then very slowly start giving it more and more throttle, all while driving it. Go all the way from 1/2 throttle to full throttle, very slowly.

If, at any time, it starts "bucking", as if someone else were driving, and they were giving it that amount of throttle, then slamming on the brakes for 1/4 a second each time, repeatedly, you'll know for sure that that's what the problem happens to be. Good luck.

 

[Zenmervolt]

Originally posted by: myocardia
That car needs a new throttle position sensor. None of the other things that have been mentioned so far can cause the engine to rev, with your foot off the throttle.

That's a bunch of bull.

A faulty Idle Air Control valve will cause exactly the issues he's having; surging at idle.

I've never once seen a faulty TPS cause surging at idle. The most typical failure modes for a TPS are either faltering under part-throttle conditions or a sustained high idle.

ZV

 

[exdeath]

Originally posted by: Zenmervolt

Originally posted by: myocardia
That car needs a new throttle position sensor. None of the other things that have been mentioned so far can cause the engine to rev, with your foot off the throttle.

That's a bunch of bull.

A faulty Idle Air Control valve will cause exactly the issues he's having; surging at idle.

I've never once seen a faulty TPS cause surging at idle. The most typical failure modes for a TPS are either faltering under part-throttle conditions or a sustained high idle.

ZV

Yeah, the TPS being bad doesn't actually open anything and allow more air into the engine

The engine is revving because the actual quantity of air entering the engine is fluctuating uncontrollably and the computer is trying to bring the mixture to equilibrium and stabilize things and chasing a moving target.

 

[Arkaign]

Originally posted by: ayabe

Originally posted by: Arkaign
You guys are awesome! Best forum ever The guy selling it has already bought a Nissan Titan, so he's going to give me over the weekend to check the car over, and even tossed me an Autozone gift card with about $60 on it for any parts/misc to put towards the car, as long as I bring him the receipt. I'll see what I can get done and post back here with the results. I really want this thing to work out, it's a beautiful car, I had a 5th gen before, but it was wrecked. Too bad I can't afford another Jackson supercharger

Hehe, I thought a couple days ago you were saying how the car was too rough on you for the long commute. Or am I getting you confused with someone else?

Hehe, well I no longer have to drive 4-6 hours of stop & go traffic in the D/Fw metroplex 5 days a week I now live 1hr south of dallas, and live less than a mile to my job So, it would be fun to have another Prelude!

For loads of bumper to bumper trafficjams, there's no replacement for a quiet automatic with cold a/c and great seats

 

[lupohki]

Originally posted by: Zenmervolt
Sounds to me like the idle air control valve. Simple fix. Sometimes they just gum up a little.

ZV

For Subarus at least ... this is a common solution to the problem you described.

 

[exdeath]

Originally posted by: Arkaign
<div class="FTQUOTE"><begin quote>Originally posted by: ayabe
<div class="FTQUOTE"><begin quote>Originally posted by: Arkaign
You guys are awesome! Best forum ever The guy selling it has already bought a Nissan Titan, so he's going to give me over the weekend to check the car over, and even tossed me an Autozone gift card with about $60 on it for any parts/misc to put towards the car, as long as I bring him the receipt. I'll see what I can get done and post back here with the results. I really want this thing to work out, it's a beautiful car, I had a 5th gen before, but it was wrecked. Too bad I can't afford another Jackson supercharger </end quote></div>

Hehe, I thought a couple days ago you were saying how the car was too rough on you for the long commute. Or am I getting you confused with someone else?

</end quote></div>

Hehe, well I no longer have to drive 4-6 hours of stop & go traffic in the D/Fw metroplex 5 days a week I now live 1hr south of dallas, and live less than a mile to my job So, it would be fun to have another Prelude!

For loads of bumper to bumper trafficjams, there's no replacement for a quiet automatic with cold a/c and great seats

Theres something to be said about idling in traffic with a V8 with a lumpy cam and full 3" exhaust with the windows down just cruising and enjoying the afternoon... it's funny how insecure and intimidated ricers and guys in big loud trucks are when you're just sitting there not even doing anything.

But yeah, my left leg gets tired after a while

 

[thomsbrain]

The F22 in my old Accord (very similar to the H22 in the Prelude) did the same thing for a while, but only when it was cold. It did it for about a year or so. It sorted itself out without any active interventions on my part, though it was receiving scheduled maintenance and the part that was at fault may have been replaced without my knowing which part it was. The last 5 years I had it, it never did it again.

 

[lightpants]

Bad IAC or faulty coolant temperature sensor.

Make sure you let us know what you find!

 

[Arkaign]

Update! Mechanic cleaned the MAP (?) sensor, reset the computer, and some other minor maintenance, and it cleared up! Car runs like a dream. Unfortunately I don't have the cash together for the car yet, hopefully he keeps it for another week for me The car seems much faster than it should be for a 200hp vehicle, but maybe I've been driving my old man Buick too long

 

[Arkaign]

EDIT : Sorry DP

 

[Zenmervolt]

Originally posted by: Arkaign
Update! Mechanic cleaned the MAP (?) sensor, reset the computer, and some other minor maintenance, and it cleared up! Car runs like a dream. Unfortunately I don't have the cash together for the car yet, hopefully he keeps it for another week for me The car seems much faster than it should be for a 200hp vehicle, but maybe I've been driving my old man Buick too long

MAP is the Manifold Absolute Pressure sensor. It's used by some EFI systems to meter incoming air and deliver fuel accordingly. Serves a similar purpose to a MAF (Mass Air Flow) sensor.

ZV