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Matt, Engineer
Category: Car
Satisfied Customers: 22005
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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Having problems getting my car to idle correctly. 1991 Honda

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Having problems getting my car to idle correctly. 1991 Honda Accord LX. I've gotten so far as getting the base idle set about where it needs to be, but only if I pinch off vacuum host #21 on the intake
JA: How many miles are on the car? What size is the engine?
Customer: 2.2L, 382K
JA: Are you fixing your Accord yourself? What have you tried so far?
Customer: just a sec, let me get you the link
JA: Anything else you want the mechanic to know before I connect you?
Customer: I just replaced the timing belt, crank seal, balancer seals, water pump.
Customer: replied 1 month ago.
This is the doc I've been going by:'ve gotten as far as step 9. The only way I can proceed past here is by pinching off the vacuum hose on the intake labelled 21. This leads into a black box bolted to the firewall. With this pinched off, I could very accurately adjust the idle up and down with the straight slotted screw on the throttle body. But I get the feeling leaving this line pinched off would not be the correct way to go, and I'd rather fix the problem rather than stick a band aid on it.
Customer: replied 1 month ago.
I have not attempted to check the ignition timing as from what I've read your base idle needs to be set before you can do that.


this really sounds like there's a vacuum leak to me or the throttle valve or idle valve is sticking open

the document you've found is an excellent guide to working through the issue

have you tried step 8 ? as this will show if any of the vacuum hoses are leaking or if its leaking somewhere else. If this is OK then I'd suggest to remove the idle valve and flush it through with brake cleaner to remove any carbon deposits and also use more brake cleaner and a lint free rag to wipe out the throttle body

Customer: replied 1 month ago.
When you say throttle valve, you've referring to the actual throttle body mechanism itself? I have cleaned that out pretty thoroughly and checked it's operation. It does not seem to be catching or sticking anywhere. I also adjusted the throttle cable tension, it seemed a little slack after almost 400k miles.And when you say idle valve, did you mean the IAC? so, I did remove and spray this down with some carb cleaner, it was a little gunked up. I also removed the cover for the fast idle valve and made sure this was seated propertly (did not remove to clean).
Customer: replied 1 month ago.
Tried cleaning the throttle body some more, no change. Tried spraying carb cleaner all around the intake and vacuum lines, no observable temporary changes in idle. I made sure to spray around the intake where it meets the head, and around the fast idle and idle air control valves. I tried pinching that vacuum line again, and again the idle cleans right up.
Customer: replied 1 month ago.
Ran the system up to operating temperature again with the radiator cap off just to make sure I have all of the air out of the coolant system.


thanks for the extra information

yes with the idle valve I do mean the IAC ( idle air control)

and that rubber hose you're pinching off runs to the MAP (manifold absolute pressure) sensor its the main load sensor for the engine

so check that this rubber line is sealed at both ends with no cracks or blockages in the rubber tube

if this is OK then you may have to replace the MAP sensor with a new part

Customer: replied 1 month ago.
I did end up taking the FITV (fast idle thermo valve) off to inspect its condition. Oh boy. Not for the faint of heart. I'll be replacing it before I move forward. That's ok, I can install my new exhaust tomorrow. The debris definitely has dirt in it, and maybe some wax. Needless to say I think I'm gonna go flush the coolant system 5 or 6 times!


well thats good to find something wrong at least

so it looks like the fast icle valve was stuck open due to all the crud.

if you still have an issue after this then check over the MAP rubber line as previously mentioned

Customer: replied 1 month ago.
I actually did check that line. I blew through it both ways, and the hose still feels amazingly soft and flexible after all these years. I would not be surprised if a replacement fixes it.

If you want to test the effect of a shut fast idle valve can you clean yours up enough so it can seal or do you think its stuck open and seized solid?

Customer: replied 1 month ago.
With the condition of the bottom "bowl", I don't think that's an option. There's just not enough aluminum left to make a good seal.


just get back to me on this post

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