Idle Hunt after fuel check valve install?

[DMC-81]

Hi there,

Today I finally got around to installing my fuel pump check valve. The good news is I didn't have any leaks from the valve,:yesss: but I noticed my engine started idle hunting after the install, (and after it warmed up). This is new. What are your thoughts?

- I have DMC's new pump (love it by the way), but it didn't have the external check valve as it was installed before they discovered that it needed one. So now I have one.
- my engine is totally refreshed, with many new parts, including new small vacuum lines
- my CO screw has the permanent plug, so it hasn't been adjusted

As I said, the engine had a smooth idle (as of yesterday) before I installed the check valve, and that was the only part that was added today. Do you think the fuel pressure was impacted slightly by the valve to cause this?

Thanks,
Dana

[Chris 16409]

This is a dumb question, but did you make sure the check valve was installed on the main feed line? It's possible to make that mistake since the hard lines are the way they are.

[David T]

This is a dumb question, but did you make sure the check valve was installed on the main feed line? It's possible to make that mistake since the hard lines are the way they are.

Some amount of hunting can be considered normal. The spec is 775 + - 50 so it can be OK if it idles and fluctuates as long as it doesn't go faster than 825 or slower than 725. It should hunt less once it is warmed up. The typical cause for excessive hunting is vacuum leaks but that assumes all of the cylinders are very well balanced. Any imbalance between cylinders (valve adjustment, spark plug, compression, etc) will exacerbate hunting. Vacuum leaks primarily cause hunting by leaning out the nearest cylinder.

[DMC-81]

This is a dumb question, but did you make sure the check valve was installed on the main feed line? It's possible to make that mistake since the hard lines are the way they are.

Hi Chris,

Yes, I verified the feed outlet against the DMC fuel pump installation instructions, and ensured it was in the correct orientation.

Thanks

[Bitsyncmaster]

This is a dumb question, but did you make sure the check valve was installed on the main feed line? It's possible to make that mistake since the hard lines are the way they are.

+1
That would really mess things up if the check valve was installed on the return line. Your primary pressure would rise the the max it can since it would block the PPR return.

[DMC-81]

Some amount of hunting can be considered normal. The spec is 775 + - 50 so it can be OK if it idles and fluctuates as long as it doesn't go faster than 825 or slower than 725. It should hunt less once it is warmed up. The typical cause for excessive hunting is vacuum leaks but that assumes all of the cylinders are very well balanced. Any imbalance between cylinders (valve adjustment, spark plug, compression, etc) will exacerbate hunting. Vacuum leaks primarily cause hunting by leaning out the nearest cylinder.

Thanks David. I have 6 new injectors, seals, and correct Bosch silver spark plugs (gap checked), new engine seals, new vacuum hoses, a clean intake system, and a valve adjustment within .001 inch (I am somewhat of a perfectionist). As mentioned, it didn't hunt yesterday.

I just started it again, and the cold start problem (the reason for the valve install) is gone.... it fires right up now :bigclap:, but after it is warm, and I rev it up, it hunts.

It's not bad, but it didn't do it yesterday. I definitely installed the check valve right. I triple checked before cutting the line. :reading:

I tried to upload a video of the engine running to YouTube to share, but it failed. I'll try to troubleshoot the video issue....

Dana

[DMC-81]

Well I started the car tonight, and when it warmed up, and started to hunt, I followed Dave S's advice about the throttle stop in post 11 of this thread: http://dmctalk.org/showthread.php?603-what-causes-the-idle-hunting/page2

With just a small adjustment, it seemed to do the trick. As of today, it's hunt free. I'll monitor it over the next week to see if that resolved it.

I am really happy that my cold starting issue has been resolved with a little brass valve. Prior to the installation, the engine seemed to have lost all the fuel in the feed line, as I needed to crank it over several times before it sputtered to life. Now it starts almost instantaneously (as it should).

Just thought I'd share the update.