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Upgraded injectors + cold start = rev hang

79K views 255 replies 55 participants last post by  HofstetmSi 
#1 · (Edited)
For those with upgraded injectors, this may or may not be an issue you've experienced. I have RC 550cc injectors. Vit has RC 370cc injectors. We've both noticed an issue with cold starts, but not in the normal sense of it being hard to start the car. It's been the type of thing where the car will rev hang excessively until the coolant has warmed up to about 120 degrees. Occasionally the idle will bounce a bit and rev up past 2000 rpm as well.

Vit and I have been working together for the last few days trying to troubleshoot and eliminate this process through a variety of tuning angles to no avail. Today I finally emailed Doug at Hondata about it.

Here's the advice he gave that ended up being the solution:

"On the intake manifold near cylinder 4 you will find a cold start thermostat valve. It bleeds air from the intake to behind the injectors to help atomize the fuel until the coolant temperature rises. I am not sure if it does anything with the RC injectors, but try blocking these hoses off."

Vit decided to give this a go first and had promising initial results. I did the same and just got back from testing it after the car had cooled down. It appears to have totally and completely fixed the problem. :dancing:

Here's why:

On the OEM injectors there are little holes in the nose of the injector that allow the air from this Air Assist Valve to enter the fuel stream on cold idle and help atomize the fuel better to reduce emissions while the engine is warming up.

See the picture below and note the 2 o-rings (orange and black) and the tiny hole for air to come into the fuel stream through.



The OEM injector uses the 2 o-ring design to force the air from the Air Assist Valve to pass through the tiny hole in the injector tip. This limits the amount of air that the engine can naturally pump through this passage to a very tiny amount. So little that it won't affect the way the car runs normally.

The RC injectors have a short tip that doesn't have the second smaller (orange) o-ring. There's no small hole for the air to flow through either.

The lack of these two things has the effect of allowing the engine to pump much larger amounts of air through this passageway than was originally intended. This heavily affects every aspect of cold start idle and driving since this air has not been measured by the engine in any way. The ECU notices that it's lean so it dumps fuel, then it notices that it's rich and goes back and forth.

The Air Assist Valve closes around 120*F so after the coolant warms up to that point, the issue goes away.

How do you do this on your car?

Super simple, just block off the hose that allows air to flow into the system. As a test, I just cut the hose, stuck a couple bolts in it and taped/zip tied it to make sure it wouldn't leak.

For the long term fix, I bought some 1/2" and 3/8" vacuum plugs and a 3/8" NPT plug.








The hole you need to plug on the intake manifold. Make sure the engine is cold before removing this. If it's cold, no coolant will come out of this when you remove it.



Here's the valve removed.




Just slip one of the 1/2" vacuum plugs on the intake tube. Since this tube only sees vacuum, there's no need to clamp this one as long as it's snug.




I used a 3/8" vacuum plug on the IM side, I put a clamp on it as well. I'm paranoid since I have positive pressure coming through the IM. N/A guys won't need this since they don't have boost (I probably don't need it either, but better safe than sorry).




Here's the 3/8" NPT plug. I bought it at Autozone for $1.49 :D

For those that have trouble finding the metal NPT Plug bolt to cap off the entire valve like did at my local auto parts stores here is a link to where you can buy it online:
Dorman/AutoGrade 3/8 in. - 18 NPT x 7/16 in. long 7/16 in. square head pipe plug (090-001.1) | Tubing Cap and Plug | AutoZone.com

Autozone part# 090-001.1



For anything like this, it's best to apply some sort of thread sealant to keep the plug from leaking. I used this high temperature permatex thread sealant. Be sure to read the instructions on the package and apply it properly.



NPT plug installed. Snug it down nice and tight.



Does this apply to you?

If you're using an injector with a short tip that doesn't have a 2 o-ring setup, yes probably. But if it isn't broke, don't fix it. Only make this modification if you notice the symptoms described above.
 
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#6 · (Edited)
My ghetto fabulous fix pic. Yes I know my **** is dirty, my car actually sees use, unlike Jay's :p

Major credit to Hondata for pointing us in this direction. I started my car with 46* ECT this morning and no rev hang. It drives beautiful when it's cold again. I've actually been dealing with this since I got the 370cc injectors. I thought I was crazy and doing something wrong with the tune until every single thing I tried came up empty handed. I jumped at the first mechanical solution that Hondata offered us.

Rulo: Teegs is having this issue with his RDX's.

 
#16 ·
Man Jay, I don't know what the community here would do without you. This is an incredible thread and one that I am sure pertains to many people here. I just wish I could tap into the vast wealth of knowledge you have. I am absolutely amazed at your incredible ability to diagnose problems. Thank you so much.
 
#29 ·
Vit, you of all people I would never "toss to the curb". I know you were joking, but I want to make it clear that I have the utmost respect for what you are doing for our members. I will not forget that. If there was any way for me to make you a vendor/sponsor, I would do so. I stand in awe of people like you, mucter, BV1, b.r.i.a.n, Soxfan143and others here on the site. You guys are almost in another dimension of knowledge than use mere mortals. I so enjoy reading all of your posts. They are so informative and helpful and they are really helping me to understand my car much better.
 
#30 ·
Couldn't have said that any better Bill. I try to help where I can, but I just don't the level of knowledge these guys do. It's amazing how much they know!

The art of working on cars these days isn't what it used to be, the truly elite also have a very technical IT type background, and these guys are shining through. Epic!
 
#35 ·
I just saw this thread too.

I have been plagued with cold start issues since my RDX injectors went in (roughly same time as my FP) and I just figured my FP was to blame. As Teegs and Rulo are having similar issues on RDX injectors, I might have to do this as well.

Thanks for the legwork vitviper (Jay you already got your props from bill, so none for you)
 
#42 · (Edited)
I wonder if this is to blame for my bad fuel economy along with the frigid weather. Because my car hasn't seen boost in awhile, it may only be a cranking valve but you never know.Also Vit on your setup are you leaving the hose connected to your intake since you did cap the the top of the the valve or did you cap the the other side and intake as well?
 
#47 ·
Also Vit on your setup are you leaving the hose connected to your intake since you did cap the the top of the the valve or did you cap the the other side and intake as well?
As much as I liked my unghetto'd version (and how easy it is to do), I ultimately removed the valve since the only extra part I had to buy was a 3/8" NPT plug at Schucks. One less hose in the engine bay, and the vacuum cap I had on the valve fit the intake.
 
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