My VX idle bounces when rolling in neutral
 

I'm puzzled by bouncy idle on my 1994 VX - It idles fine most of the time, except when rolling in neutral (e.g. coming to a stop light) the idle bounces up and down between 800 and 1200RPM; as soon as the car stops completely, the idle stays steady at around 900RPM.

A bit of history on that VX:

When I got the car this past winter it had bouncing idle at all times after the car would warm up - I replaced O2 sensor, spark plugs, cleaned EGR, cleaned IACV and cleaned throttle body and checked resistance of TPS with no effect on the idle.

A few months ago I got a junk VX engine and swapped IACV - that fixed the idle when the car was not moving, but still had a problem when the car was rolling in neutral.

A month ago I picked up 92 VX that is Canadian model; it idles fine. I swapped IACV between 92 and 94 VX. After the swap - 92 VX continued to idle fine; 94 VX started to have bouncy idle when at stop AND when rolling in neutral.

Now I swapped IACVs back and 92 VX continues to idle fine; and 94 VX now idles fine at stop, but bounces when rolling in neutral, so that condition is repeatable ...

I made sure to drain air out of coolant lines when doing IACV swaps and checked coolant level. What can this be?

Thanks

Ross

I just got a vx about a month ago, and I have the same exact problem. No other problems, the car seems fine except for the fluctuating idle while rolling in neutral. 93 VX. Sorry I have no suggestions, just thought I'd chime in...

Have you checked your PCV? It's a very common problem with Honda's and Toyota's, I've had both having the same problem where the idle would bounce up and down as you are describing, and replaced the PCV and problem fixed. They are cheap too, 5 to 15 dollars.

PCV? Positive Crankcase Vent? Where is that on these motors? I know it's on the valve cover on a sbc, but can't seem to find one on this honda...

The PCV is located on the intake manifold between the 3rd & 4th injector (facing engine, right side, just on the other side of the valve cover).

Be aware that the VX PCV is merely a hollow 90deg tube with no 'one-way' valve in it (that is down by the collector box on the engine) so when they sell you one at the parts store, be sure to get the right one.

Sounds like it could be a vacuum leak (what isn't!) Cleaning is a great start to help narrow things down but isn't always a cure-all.

One idea... did you reset the ECU after cleaning? This is done by simply pulling the 'backup' fuse(should be labled as such) for about 30 sec (with the car off, of course) and putting it back in. This should be done , btw after any kind of work on any part (or system) that has an electrical connection.

One other thing that may have happened, is that after the cleaning, your Throttle body might be feeding too much air into the system and the ecu is trying to constantly adjust. Download the Helms manual (don't have a link but google it) and follow the procedure for adjusting idle. After I rebuilt my intake manifold, I had a similar problem. *Properly* adjusting the idle fixed it for me after the cleaning. As I recall (but VERIFY w/manual), the procedure is to pull the IACV connector after the car is warm, adjusting the idle to a bit below normal idle speed (normal=600rpm?). Turn the car off, reconnect everything, remembering to reset the ecu.

...more as it comes to me.

Vehicle speed sensor? Most cars will idle around 1300-1600 rpm until the car speed drops below a certain level, it is i think 20mph on my car but if your speed sensor is giving crazy readings it could be getting confused as to what idle to run at.

Since it's a 1994(not OBD2) it may not throw a check engine light for a bad VSS. It may not even have any checks for a bad VSS and wont throw a code for it.

The obd1 Hondas usually do throw a code for the VSS. I know once the connector was corroded on my VX's VSS and after I cleaned it up, the code went away.

Sounds to me like you might have an issue with a vacuum leak, or your throttle body might not be sealing properly, or your TPS isn't working correctly. GL figuring this out. I had a similar issue with my old VX motor, but with that motor it would just die while in neutral after it got warmed up making it so I always had to be on the gas a little bit.

Thanks to all replies!

I'll try swapping PCV valve with my other VX and see if that helps, then try Throttle body and then manifold assembly.

scrappy312 - I'll put an update here if I get it solved, hopefully this weekend.

Ross

Your idle is set too high. Specification is 600 +/- 50 RPM with no load on the engine.
Paraphrasing the service manual: Warm the engine up to operating temperature (cooling fan should come on), hook up a tachometer and disconnect the IACV connector... The engine will likely stall. Slightly depress the accelerator pedal and restart the engine. Stabilize the idle at 1000, then slowly let off the pedal until the engine idles. Check that all accessories are switched off. Idle speed should be 420 +/- 50 RPM, adjust if necessary (the idle adjust screw is a brass screw on top of the throttle body, next to the MAP sensor). Switch the ignition off, reconnect the IACV and remove the BACK UP fuse from the under hood fuse box for 10 seconds to reset the ECU. Restart the engine and let it idle for 1 minute under no-load conditions. Idle speed should be 600 +/- 50 RPM.

Only reset the idle using an engine analyzer or a timing light with a tach on it. Your in-car tach is not accurate to what you need it to be. The tach in my tracker reads 6500 rpm when the engine is actually at 6750. Large margin of error.

An update on my VX idle saga.

This week I was going to swap components from my 92 VX to 94 VX till I figure out what was causing my idle to bounce when rolling in neutral on 94 VX. Just before doing the swap I took 92 VX for a test drive and found that it's actually doing the same thing when I turn AC off!

I then checked 94 Civic VX and it also idles fine with AC on when rolling in neutral. Basically they both have the same condition:

Idle at stop - with A/C ON and OFF - Fine
Idle while rolling in neutral - with A/C ON - Fine
Idle while rolling in neutral - with AC OFF - slowly (5-10 seconds) bounces between 900 and 1200 RPM

So am I on a wild goose chase here?? Should I spend any more time trying to troubleshoot this, or did they just came like this from the factory?

Also both cars idle at around 900 rpm at stop, is this normal?

Thanks
Ross

Another update - I'm still having the same problem with bouncing idle when rolling in neutral with A/C off.
Now, about a week ago I forgot to turn off my lights and had to jump start the car - while driving with discharged batter I noticed that my idle was steady when rolling in neutral. Weird. It started bouncing again when rolling in neutral after battery got charge. Any ideas?

Thanks
Ross

A bit of shot in the dark but have you made sure that all your grounding cables/straps have good, clean connections? I suppose this should also include *all* electrical connections while your at it.

I've use a product called "DeoxIT 5" by Caig Labs. I've found it to be a stellar product.

The testing you've done seems to point to a more systemic issue than a singular component problem (tho you could have vacuum issues on both cars)

Couldn't hurt to go over the electrical connections. Easy and cheap and a good thing to do in any event.

Darkelf, thanks for your suggestions, may be it's the ground issue, or vacuum, I don't know, probably something I should check out.

I was just taken aback when I found that both my VXes have the same problem - a Canadian model VX 92' with 140K, and US 94' with 260K - what are the chances. And it's such an intermittent and particular problem - only when rolling while in neutral; and as soon as the car stops, they idle fine (and also idles fine all the time when A/C is on) . Since both of my cars do this, I was suspecting that this is a common problem with a common solution. But based on the feedback I received so far that does not seem to be the case.

Thanks
Ross

I am also experiencing this problem. The A/C in my VX was removed so i cant test that and I haven't gotten around to replacing my PCV, but i have noticed that while my car is stopped and i pump the brakes the RPMs drop then jump quite a bit. Id like an update on your findings if at all possible.

900 RPM is too high and what's happening is your ECU is increasing the idle even further when you're above 7mph. Well the combined effect is that it's so high you're actually hitting the deceleration cut off point.

Get it to idle at the factory spec (600 +/- 50 RPM). Could be a vacuum leak from a hose or gasket, timing advanced too far, throttle not closing all the way (dirty, stop screw adjusted improperly, or throttle cable too tight), stuck IACV (take it off and squirt it out with some carb cleaner), or improperly adjusted idle throttle body screw.

Don't forget after any of these repairs you'll need to reset the ECU by pulling the 7.5A fuse under the hood.

ok loping (varying) rpm while coasting in neutral is somewhat normal, gotta remember that the fluid is still spinning around in there and air is still being blasted thru the radiator (cooling it since engine is at idle). could be a number of factors but really as long as it goes back to normal i wouldnt worry unless the loping is severe (to the point of stalling/high reving).

at idle when you press the brakes (hence power brake booster is being used, using alot of the engine vaccumm) it puts a strain on the engine hence makes a different noise. much like when you turn the wheel in a power steering car all the way and try to force it it makes that errrrrrrrrrrrrrrrrrrrrrrrrrrr noise and bogs the engine down.

VetteOwner, I don't think it's normal - haven't seen this on any other car. It's not too bad, except when I get stuck in stop and go traffic. Also, my idle is a little high, now it's at ~1100 rpm all the time, probably vacuum leak somewhere, haven't had the time to trace it though. Ross

yea 1100 is high, id replace all the vacum lines anyways as im sure thier dryrotted

I had idle problems with my first VX motor (had 238k on it) and I found it was a sticking throttle body that needed to be lubed and cleaned. I also found the throttle cable was adjusted wrong. To me it sounds like it is either a vacuum leak or the throttle body/throttle cable sticking.

yea age and carbon build up will do those things to any car, clean it out (run some seafoam thru the gastank) or open the throttle and scrub with soem seafoam on a toothbrush or somehting.