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What have I done to myself? 99 Accord won't go

Ok, did an intake manifold cleaning on my 99 Accord the other day, and now I have problems…

Long story short, some time ago my car started to have a very rough idle. Took me awhile to find the problem. Turned out the air hose, between the filter and throttle body, had developed a big hole on the underside. So I replaced it and while I was at it used some throttle body cleaner, spraying into the intake manifold chamber. Boy did that help! Lot of junk blew out the tailpipe. The car ran much better, but the idle could still be a little rough. I ran it that way for awhile, and finally the other day decided there was probably still some junk in there and it was time to really clean it out. So I took ff the throttle body and intake manifold housing. There was a lot of junk in there! The small intake on the passenger side was completely clogged shut. (I understand this isn’t surprising on these cars.) I used a bunch of rags and 2 cans of Valvoline throttle body cleaner and a lot of elbow grease and cleaned it out. I was thinking to myself that whatever I didn’t get with the rags flushed through OK, but I didn’t want to take the whole intake manifold out.

So I put it all back together. (I took a video before I started, so I’m pretty sure it’s all back together right.) But now I have problems. The car starts. It idles fine mostly, but can be a bit rough. But the trouble starts when I put the pedal down. If I do it in Park, well, if I push it quick the engine just dies. If I push it slow, the engine revs then quits, revs and quits, over and over. Similar if it’s in gear. If I push it hard, the engine stalls. If I push it gentle, it revs up, but chugs along a bit.

I’ve run some seafoam through it, thinking it was clogged up with all that junk, and sure enough, lots of smoke comes out, but nothing is improving so far. What have I done wrong? (please don’t say “everything”!)

Suspecting a vacuum leak. Going to take it apart and recheck everything…

Does this car have a Mass Airflow Sensor (MAF)?
If so make sure it’s clean.
You could have inadvertently moved some crud onto it.

Definitely redo your work. Clearly you have either something restricting fuel flow or a sensor problem. Did you use a new gasket? Did you damage the MAP sensor? Throttle Position Sensor properly plugged in?

This is a crapshoot, but perhaps one of the attached will stimulate an idea.

Taking it apart, already noticed the the throttle body gasket didn’t separate cleanly, it tore around the studs, so I’m taking it off…scraping it off, actually, it’s stuck pretty good.

I should probably clean the sensors while I’m at it, certainly could have gotten some junk in them. Think I saw something for that at auto supply.

Also beginning to wonder if I should have replaced the gasket under the throttle body housing. Looked like it was metal so I just put it back on, but maybe it needed replaced?..

Nice work.
Let us know how you make out.

4cyl or V6? How many miles? Read lots about carbon in intakes and how most cars never seem to run better after cleaning. I assume this is your egr ports and so on? Did u remove egr? Clean the egr ports and replace gasket?

4cyl, about 175k miles. Just figured out the EGR business…it was the EGR port that was completely plugged up, so I’ve now removed the EGR valve to thoroughly clean the port. Cleaning the valve, too.

Local store didn’t have throttle body gasket, so I still need one to put it all back together…

Haven’t been able to find any info about the gasket between the throttle body housing and the intake manifold. It’s metal, so I would think it’s reusable, but I’m not positive. I’ve done lots of searching but can’t even find what it’s called!

The link I provided has the part number.

I looked at your links, I don’t actually see the gasket I’m talking about. It’s underneath the big thing, called the plenum? Here’s a picture, you can see the gasket on top:

Not actually a picture of my car, but mine looked pretty much like that.

edit: what IS that big thing called? I’ve been calling it the throttle body housing, others have called it a plenum, and I’ve also seen “upper intake manifold”.

Upper intake manifold and plenum are the two terms that fit. For my Toyota Supra, I needed to order an upper intake manifold gasket set to get the plenum gasket I needed to do a throttle body cleaning. It also had a throttle body gasket that I needed, and the lower manifold gasket, which I didn’t.

I just checked at, and you need to get an upper intake manifold gasket set for that metal plenum gasket. At $12 for Fel-pro, it will save you a ton of aggrevation. Throw in a throttlebody gasket for about a buck or two, and that should seal it all up.

I don’t know its official title. I’d be inclined to call it a plenum.

The dealer’s parts department (NOT the service department) should be able to print you an exploded view drawing as well as a parts list.

Ok. Took everything back apart including the EGR valve this time and put it all back together with a new throttle body gasket. Didn’t get a new plenum gasket (that does seem to be what it’s called) because no one had one in stock. And it works! New gasket probably helped, and I torqued everything down a bit harder. And I did get quite a bit more gunk out.

I drove it around a bit and it all seems to be fine, except for a slight idle issue. The idle is running high, at 1200 RPM. I can only guess that it’s because while I cleaned out the EGR valve, I didn’t open it up and clean it out. But at least it’s a smooth 1200rpm! None of the slight chugging that was happening before.

When I cleaned the throttle body on my 88 Accord at 190k miles the idle was a little higher.
With that vintage there was a way to reset the “base idle”.
Removing the crud around the butterfly valve let’s more air get by when its nearly closed at idle.

Congratulations on a job well done. I tip my hat to you.

upper intake manifold or plenum arree both good names. throttle body has the butterfly in it. try removing the map sensor and make sure the passage is clear of junk. if you got cleaner in the sensor it could have damaged it.

Thanks for the kind feedback y’all. Sometimes I feel like a bungaling amateur doing this stuff.

Quick update: A little more driving and things smoothed out even more. Idle is down to 700 which is normal. The car goes much better now, especially at freeway speeds. I remember when it was new I was surprised at how easy it was to speed given that it was just a 4 cylinder. Today I was having real trouble keeping it under 70! Just a little pressure and it was going up to 75. Before I did this clean out, I had to bury the pedal fairly deep to get up to those speeds.

The idle has begun to “shudder” again. I suspected it was because I didn’t open up the EGR valve to clean it, and just read my code, P1491…yep, EGR valve lift. I’m going to have to take it out and clean it. (New ones are $155.) But I think when that’s done I expect her to drive like new! Thanks again for all the support here.


OK, Final Update, should be useful if anyone else is doing this procedure.

The shuddering idle doesn’t appear to have been caused by the EGR valve. I’m fairly certain that valve is not working, but I’ve left it in place and fixed the shuddering. The problem was air in the coolant. Disconnecting the throttle body/plenum requires disconnecting several coolant hoses, and I hadn’t gotten all the air out. There’s a bleed valve for the radiator system just in front of the throttle body. I used that to get all the air out, and the idle is perfect. Feels like a new car. :slight_smile: