View Full Version : Bouncing Idles - 1995 SLS Northstar

07-03-13, 01:53 PM
The idle bounces from almost stalling to 1500 or maybe a bit more. The engine will frequently die when slowing down to make a turn or stopping at an intersection. I changed the TPS and that almost completely stopped the engine stalling, but the surging idle continued. So this morning I pulled the cover from the top of the engine and pulled the hose that connects to the PCV valve and the idle became perfectly steady at what looks like around 600-700 rpm. The PCV valve looked fine, and I had my doubts that that could be the real problem, but I drove to the auto parts store to buy one, leaving the PCV valve hose disconnected and the car ran great with no idle or stalling problems. The new PCV valve didn't solve the problem, however (no surprise). The idle still bounces wildly with the PCV valve connected.

I really don't know what to look for now. Correct me if I'm wrong, but it seem unlikely that it would be a vacuum leak if creating a bigger one by unplugging the PCV valve hose makes the engine run better.

Edit: after thinking about it a bit more, I'm wondering if it could be the map sensor. I believe it plugs into the manifold right next to where the PCV hose plugs in.

Edit 2: The problem was a leaking fuel pressure regulator.

07-03-13, 02:23 PM
It most likely is a vacuum leak, but start with a good TB cleaning.

07-03-13, 07:58 PM
Ranger, thanks for the suggestion. I didn't find any vacuum leaks and after a thorough cleaning of the throttle body the problem persists.

07-04-13, 08:38 PM
Ok, I disconnected the vacuum line to the fuel pressure regulator and reattached the PCV line and the car runs perfectly. I'm thinking this means there's a leak in the pressure regulator and fuel is being sucked into the manifold, causing the car to run rich. I haven't bought a fuel regulator yet, but I'll report back when I have results.

07-04-13, 09:49 PM
If when you pulled the vacuum line off the FPR (at idle), fuel leaked out of the nipple, the FPR is bad and needs to be replaced. If no fuel leaked, it's OK.

07-04-13, 10:23 PM
I didn't see fuel at first. I had been thinking that the fact that the engine idled well after opening up a huge vacuum leak implied that it was running rich and when I pulled the pressure regulator vacuum hose and it stabilized all the pieces fell into place and I got excited and shut off the engine and came inside to price fuel pressure regulators. I've since let it run a bit and there's a good size stream of fuel coming out of the vacuum hose, but I'll wait until I get a new regulator installed tomorrow before I call this case closed WRT the idle bouncing.

07-05-13, 10:25 AM
I think it is a safe bet to call it case closed. There should be NO fuel at all present from the FPR nipple at idle. When they fail, generally you fill find a trickle maybe. A steady stream is a catastrophic failure. That puppy must have been running extremely rich and your fuel mileage must have been atrocious.

07-05-13, 04:50 PM
Yeah, case closed. After replacing the fuel pressure regulator it idles steady at around 650 or 700 RPM, judging by the in-dash tach.