Rough idle

[Pomorza]

Hey guys

Since about six months ago I've had issue with rough idle when restarting my car after a long drive. By long I mean long enough to get the car to operating temp for a bit. Symptoms are these. The engine won't be able to hold idle steady and when I give it throttle I get backfiring in the intake or right at the manifold.

I originally though the problem was my injectors as three of the six were leaking externally, so this past weekend I replaced them with brand new ones. (there's a thread someplace on here about it). This didn't really solve my issue at all. To be honest it seems as if it got worse at times. My fuel pressure at idle sits at about ~31 and when I give it throttle (rev to about 2k right quick) it'll pop up to ~35.

I've been told that this is a common problem with 280s as the fuel rail gets rather hot and the fuel vaporizes in the line. Is there any way I can rid myself of this issue.

Jan

[cozye]

The vapor lock issue is a warm start issue that is common, but only if fuel rail pressure drops while sitting. You don't get vapor lock when the fuel pressure is at 32 psi normally, and you certainly wouldn't get it while the car is running as the fuel isn't in the rail long enough to get hot. The fix for the vapor lock issue is to find and fix any leaks in the fuel system so that the rail maintains pressure while shut off.

What seems to be common on the 280 however is a lean running condition, theorized to be caused by drifting components in the ecu. They lean out over time. If this is your issue, the reason you notice it after the car warms up, is that during the warm up period the ECU is in "enrichment" mode and gives a lot more fuel than it does after it reaches operating temp.

I've had this problem, Sarah had this problem, and there are traces of threads here and there. If you go this route though, you have to rule everything else out first, as the fix may just cover up some other symptom (like a vacuum leak) etc..

So, what is your vacuum readings how? Have you verified that you do not have any vacuum leaks?

The fix is easy if this is your issue.

[FastWoman]

^ yep!

Eric and I have some pretty long threads going about these issues. My long and meandering "purs like a kitten" thread chronicles what I did to straighten out my engine. Jenny has a very long thread going about her '76, and she's in the process of doing what Eric and I did.

I don't know how new injectors would compare to old injectors. I suppose the old injectors could have deposits in them, but they might also be worn. Whether the end result would be more fuel or less is hard to say.

In short, what Eric and I did was to put EVERYTHING right (to spec) with the fuel/intake system and then to correct our lean-running situations by adding resistance to the coolant temperature circuit.

[Pomorza]

The vapor lock issue is a warm start issue that is common, but only if fuel rail pressure drops while sitting. You don't get vapor lock when the fuel pressure is at 32 psi normally, and you certainly wouldn't get it while the car is running as the fuel isn't in the rail long enough to get hot. The fix for the vapor lock issue is to find and fix any leaks in the fuel system so that the rail maintains pressure while shut off.

What seems to be common on the 280 however is a lean running condition, theorized to be caused by drifting components in the ecu. They lean out over time. If this is your issue, the reason you notice it after the car warms up, is that during the warm up period the ECU is in "enrichment" mode and gives a lot more fuel than it does after it reaches operating temp.

I've had this problem, Sarah had this problem, and there are traces of threads here and there. If you go this route though, you have to rule everything else out first, as the fix may just cover up some other symptom (like a vacuum leak) etc..

So, what is your vacuum readings how? Have you verified that you do not have any vacuum leaks?

The fix is easy if this is your issue.

I will do all test (electrical and such) this long weekend. Here's two issues I wish to discuss. 1. I have a leak at the sending unit of my Z. I'm not sure how this would affect the car but I'm going to fix it nonetheless. 2. Due to emissions in Arizona, I've had my car run through a exhaust analyzer a number to times by my mechanic. I've asked him multiple times if I was running lean at any time and he stated that he has the car set to run exactly as the manual says and even a tiny bit rich as the timing is advanced to ~17.

Jan

Edited January 14, 2011 by Pomorza

[cozye]

have you tried backing the timing down and seeing what the results are ? Odd that the mechanic would state that it wasn't running lean, however if you are backfiring through the intake, that would seem to indicate a lean condition.

[Pomorza]

have you tried backing the timing down and seeing what the results are ? Odd that the mechanic would state that it wasn't running lean, however if you are backfiring through the intake, that would seem to indicate a lean condition.

I have not tried to back it down. He reassured me a few times as I asked him to make sure it wasn't running lean. It only seems to do it when i restart it and only for a few minutes. I just drove to wallyworld and parked for about 20 min. Came out and started. Again had the rough idle condition and backfiring. But after a few seconds (before I even got out of the parking lot) it cleared up and the car drove fine.

Jan

[cozye]

ok, so I'm confused a bit. I misunderstood. So you are saying that the problem happens only after you have driven it for a while, got it hot, then stopped and parked it for a few minutes, then start it back up and begin to drive? Then after you drive it for a few minutes it clears back up?

Yes, this would be the hot restart issue. You are loosing pressure on your fuel rail. There are only so many ways to loose pressure on your fuel rail.

leaking injectors

leaking cold start valve

leaking fuel pressure regulator

or a leaking check valve

If you want to be sure, put a fuel pressure gauge on your car, prime the fuel pump, then let it sit overnight. It should hold fuel pressure.

Mine has the hot start issue, although I've resolved several of the leaks. My cold start valve was leaking only when hot. My fuel pressure holds much longer now, but I've still got a leaking injector or slightly leaking check valve.

If your fuel rail doesn't loose fuel pressure, the gas won't boil as easily in it.

[Pomorza]

ok, so I'm confused a bit. I misunderstood. So you are saying that the problem happens only after you have driven it for a while, got it hot, then stopped and parked it for a few minutes, then start it back up and begin to drive? Then after you drive it for a few minutes it clears back up?

Yes, this would be the hot restart issue. You are loosing pressure on your fuel rail. There are only so many ways to loose pressure on your fuel rail.

leaking injectors

leaking cold start valve

leaking fuel pressure regulator

or a leaking check valve

If you want to be sure, put a fuel pressure gauge on your car, prime the fuel pump, then let it sit overnight. It should hold fuel pressure.

Mine has the hot start issue, although I've resolved several of the leaks. My cold start valve was leaking only when hot. My fuel pressure holds much longer now, but I've still got a leaking injector or slightly leaking check valve.

If your fuel rail doesn't loose fuel pressure, the gas won't boil as easily in it.

That is exactly what I'm saying mate. I drive the car, get it nice and warm. Park and come back in say 20 min. Restart and have horrid idle as explained. I drive off and it clears up. The injectors are brand new and I don't think they are leaking. Will test the cold start valve this weekend.

[cozye]

That is exactly what I'm saying mate. I drive the car, get it nice and warm. Park and come back in say 20 min. Restart and have horrid idle as explained. I drive off and it clears up. The injectors are brand new and I don't think they are leaking. Will test the cold start valve this weekend.

I noticed on my cold start valve I tested it, and couldn't see it leaking. But if I drove it and got it hot, then parked it for a few, I could see down in the cold start valve hole that the manifold was wet from gas. I'm not sure you will be able to test the cold start, fuel pressure regulator, or check valve effectively. One way would be to clamp off the fuel lines going to those items, but my results when I did that were inconclusive. There is definitely a small leak down on your fuel rail somewhere.. The factory check valve is located on the fuel pump outlet, but is NLA.

[Pomorza]

A question for you guys.

If one was to unplug the cold start valve and start the car should it run differently? (when cold)

Jan

[Jennys280Z]

I will guess that the CSV wouldn't come on for the 8 seconds after startup like it normally does. But if it's leaking, I'm not sure whether or not unplugging it is going to fix the problem.

[cozye]

unplugging it would not fix a leak more than likely, It's just an injector basically and will be under the pressure of the fuel rail constantly.

If the car runs differently with the cold start valve disconnected, you may be looking at the thermotime switch ? Without referencing the FSM off the top of my head, I believe it is what triggers the CSV, and it has a thermal switch built in so that it does not trigger the CSV after the car is warm.