1990 Honda Accord 90 Accord up and down idle

MTMTNTOP

MEMBER
1990 HONDA ACCORD
4 CYL
2WD
MANUAL
248,000 MILES

I have read alot of posts on this problem. Mine seems a little different. Idle goes to 1500 then 1000 when warmed up and cold. Cold its a little higher rpm. I have 2 of these accords so swapped the iac unit. No change. Checked coolant level, ok. Checked the hoses to the iac unit and had coolant in those small hoses. Checked for codes, none showed up. Car runs fine other than this. Really have not had any problems even with this high mileage. Thanks.

Thursday, April 30th, 2009 AT 6:29 PM

3 Replies

ATKAPARKING

MEMBER
64 POSTS

On these cars I believe there is a large brass screw recessed into the intake plenum--this controls air/fuel mixture. The o-rings on these go bad--if your car has this setup, just cover the hole with your thumb to see if your erratic idle goes away (bad o-ring creates a vacuum leak). You can either replace o-ring if available from dealer or fill the hole with silicone. Also worth while to check for other vacuum leaks on this system--use carb cleaner (flammable kind) and spray each section of line. Good luck

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Thursday, April 30th, 2009 AT 6:36 PM

MTMTNTOP

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3 POSTS

I do not see that brass screw. Can you give me an idea of where to look? Thanks

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Thursday, April 30th, 2009 AT 6:47 PM

KHLOW2008

MECHANIC
41,814 POSTS

Hi mtmtntop,

The idle adjust screw is located on upper portion of throttle body. It could have worked itself loose if any adjustment had been made before. The diagram is not the exact one for your vehicle but it shows the location.

When idle speed increases, it is caused by excessive air getting into the intake manifold so you should check :

1. Clogging of the coolant hoses of IAC would result in same problem. Coolant might be present, but did you test if they were clogged? Remove the hose and apply low pressure to each side and reconfirm.

2. Check the Fast idle valve, next to IAC. Remove the air cleaner hose. Warm up engine and with finger, test the upper port of the throttle body for vacumn. If vacumn is high, the Fast Idle valve is not closing fully. These valves tends to work themselves loose. Remove the 2 screws holding the cover and check the white plastic position. It should be seated with about 2 threads visible at the side of the internal housing.

3. Check PCV on top of valve cover.

4. If the brake pedal seems stiff at times, check for vacumn leakage at the seal between master cylinder and booster.

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Thursday, April 30th, 2009 AT 8:28 PM