Rough idle

WestCoastSVX · #1 12-01-2008, 09:27 PM

Since my engine reseal I've had a bit of a rough idle.

But I didn't really notice how bad it was until getting stuck in stop and go traffic this afternoon. At a stop the idle goes up and down to the point that one mintue it feels like it is about to die, then it will almost lurch forward if I'm not on the brake very hard. The RPMs seem to stay about the same, it's not like the needle is dancing up and down during this. Also while coasting at very low speed in bad traffic the same thing was occuring.

I've got a new battery in there and during the up and downs I don't notice the headlights dimming or anything so I don't "think" it's anything electrical related. I could be wrong though?

The stop and go low speed traffic driving also made me realize I have a very wicked 2-3 flare.

want-a-fast-svx · #2 12-01-2008, 10:06 PM

Vacuum leak maybe? Just a thought.

TomsSVX · #3 12-01-2008, 11:02 PM

vac leak would be my thoughts too. What all did the shop replace when they took the manifold back out??

Tom

longassname · #4 12-02-2008, 06:41 AM

could be a stuck open egr too. engines don't idle well on exhaust gas instead of air

WestCoastSVX · #5 12-02-2008, 08:33 AM

Quote:

Originally Posted by TomsSVX

vac leak would be my thoughts too. What all did the shop replace when they took the manifold back out??

Tom

They just replaced the punctured coolant hose, the one with a pin prick right in the middle under the manifold. Could they have forgot to hook something back up? You replaced almost all the vacuum lines didn't you? That was the worst part of the emergency repair, was that I had to have a random shop that I was having do an alignment mess with the manifold.

Although you'll remember it had an uneven idle as soon as we got it all back together, but at that point you figured it was due to the poor battery.

kwren · #6 12-02-2008, 12:45 PM

uneven idle... poor battery????

SoCal LS-L · #7 12-02-2008, 01:38 PM

Knowing random shop mechanics.... the guy probably forgot to hook up a vac line or connected them wrong. You can try going under the manifold with a mechanics mirror and a flashlight... you might get lucky and find a disconnected hose, but if it happened immediately after they reinstalled the manifold... you have a vac hose connected wrong or they didnt install a new manifold gasket (maybe couldnt find one?). The point is.... you have to go back over whatever they messed with.... the answer lies within. I have worked with techs who spend 2 hours trying to diagnose a battery light and overheating problem, relieved when I walk up and point out the broken drive belt

. Good luck!

WestCoastSVX · #8 12-02-2008, 02:38 PM

krwen - brand new battery has been in there for a couple weeks.

Yeah, as much as I am nervous about the job the shop guys had to do (what had happened was I went in for an alignment, and one of the coolant hoses burst when they drove it into the shop - I had noticed something wrong a couple days before that, so no, they didn't sabatoge it, just bad timing), to be fair the rough idle was happening the week before they did it, Tom even noticed it when he put my engine back together but at that point I had a bad battery so we couldn't be sure if that was the cause or not.

I'm going to call Tom tonight and he's going to walk me through a few things and we'll see if we can get this figured out. I'll report back.

TomsSVX · #9 12-02-2008, 03:08 PM

Quote:

Originally Posted by kwren

uneven idle... poor battery????

Poor voltage can have many effects on the ECU. Most notably on injector response time. When this happens, the injector will take longer to open (due to less voltage) and actually lean the car out. The ECU will see a lean condition and take action to richen it up... And this the cycle continues. Also it effects the sensors like throttle position and the like that all depend on a good +v to read properly.

Tom

kwren · #10 12-02-2008, 04:04 PM

Thanks for the explanation Tom.
I was thinking that the alternator determines the voltage instead of the battery when idling.
Just trying to learn as much everyday as possible!
Keith

oab_au · #11 12-02-2008, 04:32 PM

Quote:

Originally Posted by TomsSVX

Poor voltage can have many effects on the ECU. Most notably on injector response time. When this happens, the injector will take longer to open (due to less voltage) and actually lean the car out. The ECU will see a lean condition and take action to richen it up... And this the cycle continues. Also it effects the sensors like throttle position and the like that all depend on a good +v to read properly.

Tom

The ECU accounts for applied voltage by adjusting the pulse on time to suit the slower pick time.

The voltage used for the vital sensors is a regulated 5 volt, so that they are immune to variations in supply voltage.

Harvey.

TomsSVX · #12 12-02-2008, 06:28 PM

Quote:

Originally Posted by oab_au

The ECU accounts for applied voltage by adjusting the pulse on time to suit the slower pick time.

The voltage used for the vital sensors is a regulated 5 volt, so that they are immune to variations in supply voltage.

Harvey.

You are correct, but I am not sure what the lower threshold for the injector adjustments are. i would imagine somewhere around 8-10v but I could be wrong. It does affect the idle and performance of the car if the battery is bad, so troubleshooting something like this with a bad battery can sometimes be a futile effort

Tom

kwren · #13 12-03-2008, 04:31 AM

If the battery is suspect might be a good idea to switch a good battery to it and see if the battery was the idle problem.

would just take a few seconds!

TomsSVX · #14 12-03-2008, 11:35 AM

Quote:

Originally Posted by kwren

If the battery is suspect might be a good idea to switch a good battery to it and see if the battery was the idle problem.

would just take a few seconds!

He already got a new battery in it so that as a suspect is gone. I walked him through a couple troubleshooting excercises yesterday but couldn't find anything. I will be send a care package today with a couple things that might also help us find any issues... more to come

Tom

WestCoastSVX · #15 12-03-2008, 11:50 AM

Yeah what Tom said.

We determined it wasn't the TPS or a vacuum leak anywhere I could get to at least.

Tom - last night I was wondering if the alternator was on it's way out if this could be affecting things? But if so, the new battery surely would have been drained by now, or at least not read as it good when I tested the ground last night?

This morning I was at a stop light and the idle didn't fluctuate, but it just seems really low, like it's about to die. It's worse when it's just been started, but even after it warmed up 20 miles down the road it still just seems like if it sat there in D long enough with no gas pedal, it would die. Reads about 500 rpms at idle, and like I said, no fluctuation, at least this morning.

Eagerly awaiting the care package and more troubleshooting.