my '84's idle ssurges a little when the engine's hot

[450knotOffice]

(This is an update to an old thread I started a couple of years ago that JeremyD pointed out I never resolved. Problem resolved and result posted on page 2.)



Guys, I'm sorry to bring this up again on this board considering this topic has been discussed at length but I'm not finding any good answers for my specific problem by doing a search. So let me be very specific.

I was out driving my car around Venice Beach / Santa Monica this past Saturday and got caught in quite a bit of weekend traffic. The oil temp gauge got a bit warmer than I've ever seen it before (normally it rides a tick below the first white line but this time it was a tick above the first white line). I started noticing that the car would buck slightly at a neutral throttle position (neither accelerating nor decelerating) at low RPM. When I would push the clutch in the idle would drop to maybe 900 RPM then quickly bounce back to 1,100. It would then bounce back and forth for maybe 5 to 15 seconds before it would finally settle down. I also noticed that it was idling about 50 RPM higher than it normally does.

Again, this only happened when the engine was really hot (stop / go traffic on a warm day). I drove the car again yesterday and saw normal temps and no idle surge.

So, what gives? Why does my car only seem to do this when it gets really warm?

Help!?

-Scott

[mede8er]

I had this prob....got an air fuel analysis (sniffer) and leaned it out a little...no more prob....

[nhromyak]

Mine did this before I got the two-wire update for the CHT.

[450knotOffice]

OK. Here's the latest update.

I hooked up an RPM meter to the engine and disabled the ICV by running the jumper wire between test ports B and C as instructed elsewhere. My RPM went from about 750 and fluctuating about +/- 40 to 860 and rock steady.

I tried numerous times to get the idle indications to match by adjusting the screw on the throttle body but to no avail. Whenever I disconnected the jumper the idle would drop about 75 to 100 RPM and become a little jumpy again. As soon as I'd put the jumper back in place the RPM's would climb back up slightly and steady out perfectly.

Should I take the ICV out and clean it? Does a failing ICV exhibit these characteristics?

-Scott

[john walker's workshop]

1100 is way too high for a carrera, and will cause the idle to surge up and down repetedly. 800 is the suggested idle speed.

[450knotOffice]

Normally the engine idles at around 800, but during this episode it would bounce around from 800 to 1100 then finally steady out at about 900 (which was higher than I was used to seeing). It only did this when the engine was a bit warmer than usual due to a hot day and stop / go driving.

-Scott

[nut11]

I had the exact same problem. I gently wacked the ICV (loosen the innards) and then sprayed WD 40 into both hose connecting ends. It's been 18 months and no searching idle. Maybe I got lucky but good luck to you.

85 Carrera Coupe

[john walker's workshop]

900 is still pushing it. try lowering it a bit still.

[nhromyak]

JWW could this not also be an indication that his IDLE switch is not working???

IIRC - the Idle switch when open, will cause the computer to go into "off-idle" and bring the RPMS up to 1100 - correct?

[john walker's workshop]

possibly.

[stlrj]

Retorque your intakes and see if that helps.

-Joe

[450knotOffice]

The only time I've had this issue come up was yesterday afternoon when the oil temp had gotten a bit warmer than usual due to being stuck in stop and go traffic on a warm day.

The car has always idled at about 800 RPM or so and has never exhibited that annoying little rrrumph, rrrumph, rrrumph swing from 800 to 1100 RPM before. It had never idled as high as 900 RPM until yesterday and it didn't do it at all today. Today it was perfectly behaved all day (normal idle RPM and no surges) but I kept the temperature in it's "normal" spot - one needle width below the first hash mark. Yesterday it was one needle width above the hash mark.

I did disable the Idle Control Valve today in order to check the idle RPM and it immediately increased about 75 RPM or so and steadied out completely. Normally, the idle varies a bit (maybe 20 to 30 RPM from baseline) but with the idle control valve taken out of the loop it sat there steady as a rock - it didn't move even 5 RPM.

Does it appear to you guys out there that I am having a problem or not? Is what I described normal when the engine heats up a little more than normal? Does it seem that I am having a problem with my Cylinder Head Temp sensor? Or do you think it may instead be my Idle Control Valve? How about my O2 sensor?

It only acted up when it got hot. Otherwise it runs really well - strong and smooth.

-Scott

[nhromyak]

Originally posted by 450knotOffice

Does it appear to you guys out there that I am having a problem or not? ....

It only acted up when it got hot. Otherwise it runs really well - strong and smooth.

-Scott [/B]

Do YOU think your having a problem?

I would say maybe, but it depends on what you expect from your 16 year old car.

[kqw]

When was the last time the O2 sensor was replaced?

[450knotOffice]

To me it's a minor issue but one that should be fixed if in fact it is an abnormal condition. The problem is I have no frame of reference from which to judge whether or not this condition is "normal". I've never driven another Carrera other than my own.

This is why I am asking. I could take my car to Hergesheimers and start having them fiddle with it and pay an arm and a leg for labor, or I could ask people on this board and try to fix the problem myself. That's what I am attempting to do.

Thanks for the tips though. I'm thinking the culprit may be the Cylinder Head Temp Sensor giving bad readings at high temps.

-Scott

[450knotOffice]

kqw,

I don't know. I've only had the car about three months. Think I should change it out, just to be sure?

-Scott

[kqw]

I would do as suggest here....Check all hoses and vac lines etc. The O2 sensor on the '85 is probably around $225.00. So I would eliminate a few things before replacing it.

Torque the Intake manifold (18ft/lbs). Make sure to get all 12 bolts. (8mm allen socket)

Start the car and unplug the CHT from the holder on the left side of the engine bay. The engine should race. You have three sensors on this rail. The CHT should be the TOP connector. Tell me what happens when you disco the CHT.

When setting the idle, the engine should be at operating temperature. 800rpm is just about where you want to be. Patience....

The center boot between the left and right manifolds should be checked and cleaned.

When was the last time you did a major service? Plugs, filters, valve adj. etc...

[ckmaggie]

Scott,
I am having the exact same problem w my '85 3.2. As you said, it occurs once the car is hot and I come to a dead stop due to traffic or something. As I move forward in traffic, my idle surges when I come off the accelerator.

I took my ICV off and sprayed w carb cleaner and things inside moved more feely but the problem remains. I have heard more than once that it may be the old style CHT and I should just replace w the two wire upgrade.

I am of no help other than to say that this is not normal and I am trying to trouble shoot along w you.

Chris

[450knotOffice]

Thanks guys.

kqw, I will take your advice and check the torque settings on the manifold among other things you mentioned. I'll take a look at that CHT sensor too. I haven't even looked at it yet so I have no idea whether it's a one or two wire system. As for a major service, unfortunately I have no receipts or history on this car and as yet have not taken it in for it's major service yet.

Chris, I feel your pain. It's a little irritating isn't it.

[450knotOffice]

kqw,

I unplugged the cylinder head temp sensor while the engine was idling and after warming it up to normal operating temperature and, just as you said it should, the engine began to race. I plugged it back in after about a second and the idle settled right back down.

What does that mean?