98 civic erratic idle

Good day,
98 civic LX 5 spd with 285000kms.
After being on the highway during my 35km commute, when I pull up at a red light, my RPMs stay at around 2500. After a few seconds, or if I pump the
gas pedal, the RPMs go down to about 1200-1500.
This just recently started happening. Don't know if its related, my AC recently stopped cooling. Sounds like the compressor doesn't come on. Is there anything in the AC mechanism that affects idle speed?
Also, have recently done the following, all with OEM parts from the dealership:
Spark plugs Distributor Cap Distributor Rotor PCV valve Air Filter Front wheel bearing rebuilt all struts all new suspension components front and rear 4 wheel alignment
When I did the distributor cap, I actually had to remove the distributor so I could drill out 2 of the bolts holding the cap on. (They were rusty and broke). I was expecting the removal of the distributor to be harder, but it was pretty much idiot proof. It appears that you can only put the distributor back in the block either the correct way or 180 degrees wrong. So if you are careful, you should be OK.
Also, since my distributor had never been removed before, it was easy to put it back exactly the way I had removed it as I could see where the washers for the adjustment bolts had been before (good old corrosion at work here again). Checked the timing with a timing light after, and all is well.
Note that the idle fluctuations occured before I did the tune up work (which was overdue anyway).
I am also fairly certain the car is running pretty rich. Fuel economy is relatively poor (around 11-13km/liter or 25-30 US MPG or 31-36 UK MPG), and the exhaust pipe is pretty black. I have run several bottles of injector cleaner and Lucas Oil Upper Cylinder lubricant over the past few tanks and seen no real improvement here.
The car has great performance, when you want to step into it, it accelerates really well. However, I have also measured fuel economy on a light foot, and the mileage is about the same.
Driving pattern is mostly highway, we live about 20km outside the city.
Any ideas of what I could check would be greatly appreciated.
Thanks Terry in Winnipeg
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my suspension parts were aftermarket, not from the dealership... though I doubt that has any bearing on my idle issue.
t
loewent wrote:

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Still too high.

A switch to tell the ECU to increase the idle.

No plug wires? They don't last forever.

ECU trouble codes? your's is an OBD-2 model.

you're probably overdue for a new O2 sensor(they last ~60-100K miles),and you need to get the ECU trouble codes read;a rich condition should set a code,light up the MIL light. Perhaps a bad throttle position sensor for the idle problem?
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I agree the 1200-1500 is way too high.
No trouble codes, I have a code reader.
Plug wires have less than 100k kms on them. They are lifetime warranty bosch, and I did a continuity test and a resistance test on them, they are good.
Where is the switch for the AC? I need to test it.
I will check the TPS. any pointers?
Thanks t
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are my symptoms indicative of a MAP sensor failure?
t
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loewent via CarKB.com wrote:

I think you've got air in your cooling system. I'd fill the reservoir to MAX and observe where it is each morning. Add more until is stays at MAX each morning.
When the coolant temp sensors are high-n-dry, the computer has no idea where to set the timing / mixture.
'Curly'
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Went for lunch and this is what I observed:
RPMs = ~1800 when cold.
After driving for 10 minutes, RPMs were around 1800-2200.
I got to my lunch spot, and turned the car off. Then started it immediately. RPMs were at 600-700 (normal). Then I turned it off and went for lunch.
Coming back to work, the RPMs were back up at around 1500-1800. When I got to my parking spot, I popped the hood and fiddled with the throttle a bit. By turning the throttle I was able to get the RPMs down to 600. So I'm wondering if this is a problem in the throttle body?
As far as the coolant, all appears normal, rad is full to the top and resevoir is a little below normal. I haven't done any work on the cooling system for quite some time, so I doubt that an air pocket would develop spontaneously. I'll keep an eye on it.
t
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Maybe the throttle body has varnish built up and is keeping the plate open more occasionally? A thorough cleaning would correct that. Have you ohmed the TPS sensor? It could be getting worn at the "closed" end of the pot.

there are ohms tests for MAP sensors.

I had a CTS open on my 94 Integra GS-R. It's just a thermistor,an ohmmeter will verify it. Service manuals should list the ohms ranges for the various sensors.
I'd like to note that O2 sensors can get "slow" near the end of their life,and not set a trouble code,but still affect operation.
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Got some Throttle body cleaner, sprayed the throttle plate and tube. Also lubed the throttle spring where the cable attaches. Seems to be OK now.
Is there a way to test my O2 sensors to see if they are bad? (or slow as you put it)
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I've read that you can use a oscilloscope to check them. Never done it myself.

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Have a look at your injectors. Do you have air lines running to each injector?
How clean is your throttle body?
How old is your thermostat?
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Tegger wrote:

No air lines that I can see.

Not. I will clean it up good tonight, I did a basic cleaning to free up the spring and the throttle plate.

Original. I have a new one, haven't put it in yet.
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Then you don't have "that" kind of injectors.

Make certain that thrpottle body is squeaky clean. There shoyuld be NO traces of gum or carbon ANYWHERE, especially in the tight sports.
Use a brick or something on the gas pedal to prop the throttle plate open. Do not spray the cleaner dirtectly into the TB, bt spray it on a tothbrush and use that to scrub the carbon. Wrap an old T-shirt around a dry toothbrush and use that to wipe.

I hope it's OEM and not aftermarket.
An old thermostat can fool the ECM into thinking the engine is colder than it actually is. The ECM will then go into "warmup" strategy, feeding the engine excessive air and fuel, leading to a high idle.
Fix the simple stuff first. Don't tackle hi-falutin' things like the TPS when the problem is about 99.99% certain to be a lot simpler than that.
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Yes, its OEM. I bought about $600 worth of parts last time I did my timing belt, and the t-stat is one of 2 leftover... I also have an oil pan gasket I've been meaning to use.
Have doused the spring assembly where the throttle cable hooks up with a good cleaner, and have now lubed it using a good spray lube (no WD40!). The problem seems to have gone away.
Definitely doing the simple stuff first. The shop manual says the TPS is not serviceable... IE the Throttle body has to be replaced.... OUCH. I start with cleaning the original assembly.
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The linkage is not your problem. You should not have ANY grease there at all, as it will eventually attract dirt and begin to gum up, causing more grief.
Clean that grease off the outside of the throttle body and leave it bone- dry.
Make 100% certain the INSIDE of the throttle body is cleaned according to my previous (badly typed) instructions.
If I were you, I'd replace the thermostat anyway. It's simply too old for comfort.
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