Surging idle issue

[Dean]

Okay, I replaced the IAC valve on the TB and also replaced the Throttle Position Sensor thinking these may have gone bad when I was cleaning the TB. The Idle still won't stabilize, even at the car fully warmed up (waited till the cooling fans kicked on). I checked all vacuum ports three times and everything is connected.

When you drive it, it drives fine. But when you come to a hard stop, the idle drops below normal and car stalls out. Then if you come to a gradual stop it maintains the unstable idle and idles for a few seconds then car stalls.

I tried the idle relearn three times, didn't work for me.

I was now suspecting the device that is located and mounted on the lower set of studs that hold the coil pack, so I looked at the GM shop manual and Chiltons, and they do not explain what this device is. It has to two power leads going to it and has two hoses connected to it as well (I assume one comes from the fuel pressure regulator; and the other ties to a fuel line, a 1/4-inch rubber hose which routes to a hardline going to the back of the car). It has a third port that doesn't look like it was connected to anything before my engine teardown, it has a 90-degree rubber connector. Do not know what this may have connected to!?! I checked for any lose hoses in the back of the engine and saw nothing. Do not know if this device went bad when my car over heated!?!

I also looked at the EGR. Unbolted it from the manifold and inspected the little plugger at the bottom. It moves freely and does not stick. I put it back on and unplugged it started the car with it off, and the car still exhibited the same surging/erratic idle. So I plugged the EGR back in.

Confused!?!

Anyone have any suggestions or another method of process of elimination??

Where can I get a vacuum line schematic to make sure all my routing is okay?

Thanks!

[Doubt_Incarnate]

what car is this, year and motor?

the part you decribe sounds like a vacuum distribution valve, or the canister purge valve, or something along that line. that line going back is a vent from the tank, it shouldnt have fuel in it. and the other tube should end up going into a port on the throttle body. if it doesnt work properly, the motr will be drawing in air through the fuel tank, and it would be enough to cause an idle problem. dads 99 century with a 3100 had a prob with this, the dealer re-flashed his pcm and solved the problem.

unplug it, see what happens. if that doesnt work, take it off and plug both lines so you dont have any leaks and see what happens. if thewre is no change the problem is elswhere. but that may be sticking or not working as it should.

[tejohnson]

Re: Surging idle issue

I was now suspecting the device that is located and mounted on the lower set of studs that hold the coil pack, so I looked at the GM shop manual and Chiltons, and they do not explain what this device is. It has to two power leads going to it and has two hoses connected to it as well (I assume one comes from the fuel pressure regulator; and the other ties to a fuel line, a 1/4-inch rubber hose which routes to a hardline going to the back of the car). It has a third port that doesn't look like it was connected to anything before my engine teardown, it has a 90-degree rubber connector. Do not know what this may have connected to!?! I checked for any lose hoses in the back of the engine and saw nothing. Do not know if this device went bad when my car over heated!?!

This would be the EVAP Canister Purge Solenoid Valve. Do you have a 99 (or lower) N-Car? Should be a plastic line going to a hard line that has a green cap on it. It converts to a hard line just under the master cyl. As for the other line, it bridges with the PCV valve line and gets suction from the UIM (As mentioned above). Do you have an SES light on? Typically the EVAP purge occurs on startup, so I would be surprised if it went bad if there is no SES, and this occurs after the car is running for a while, but the plastic tube bridging over the UIM can develop prurdy bad vacuum leaks.

Funny, I just removed my EVAP Solenoid yesterday, plugged the UIM with a rubber stopper, and have had a breather where the PCV used to be. Plugged the EVAP tube at the hard line, and removed the EVAP DTC's from my calibration Now I just need to find a locking gas cap which will vent off positive pressure...

[Dean]

Surging idle issue

Sorry I forgot to mention my car is a 96 GP with the 3100.
This EVAP Canister Purge Solenoid Valve, I do have the connections made to it, but I see a third port with a 90-degree rubber fitting, which looks like it was never been used or has never had a connection to it. The reason I say this is that it has dirt/dust in it, probably a 1/4-inch up inside the fitting. I did not get a MIL to pop at all.

I think that the EVAP Canister Purge Solenoid Valve also gets the plastic line from the fuel rail pressure regulator and connects to the top of the intake manifold and then connects to the EVAP Cannister Purge Solenoid. Like I said I think this is the way it is routed. I am trying to visualize this since I am not close to my car currently.
So how do you get around the fuel pressure regulator and give it positive vacuum of you do this bypass?

[tejohnson]

I did this in order to get rid of additional vacuum lines due to the turbo setup. I would recommend you keep the EVAP system since it keeps positive pressure in the tank down by burning of the evaporated fuel.

The line that is going to the FPR should be from the MAP sensor and attached to the UIM. As far as my EVAP solenoid, it only has 2 attachments, one for vacuum, the other to the EVAP purge canister. There is a plastic end on the other side of the solenoid which is bigger than the vacuum fittings, but it is not any form of an attachment or anything. It has a traiangle shaped hole. There should also be a 2-lead connector going to the PCM from the solenoid.

Unless someone else knows better about this particular model/year, I would assume yours would be very similar to my 99 GA, could you get a picture?

[TurboGTU]

spray some carb cleaner around the TB (away from the bore) and around the intake manafold...a spike in rpm indicates a vacume leak. Don't forget to check the hose that goes to the brake booster. BTW>..how are your brakes feeling? Any LOUD hissing than before the idle mess started?
I would think the vacume schematic would be on a lable on your strut tower.

Did you disconnect the battery when changine the sensors? If you don't..the ECM will get lost at where the IAC and TB dead points are. I always disconnect it and then connect it 30 seconds later. It starts out good..and then it learns idle.
Just throwing out ideas.

[Dean]

Surging idle issue

I removed the TPS and IAC from the TB; and cleaned the IAC valve ports. Bolted the TB back in, the result was the same.

I reconnected the battery and it still no change.

As for the brake booster hose. I made sure this was connected and clamped.

Is there any chance that the MAP sensor could have gone bad? But I would still get a DTC for that wouldn't it. The MAP also would give you a lean out condiction!?!

[tejohnson]

It is possible, it would be nice to see a scan to be sure. If there were a vacuum leak at the MAP causing more of an open vacuum, you would go rather rich since it seems more like WOT (closer to 100 kPa). Unless you have a turbo/supercharger...

[ProjectMM]

I'm not sure if it would pertain to a 96, but the older 3.1 flat top motors. the IAC has to be programed with a scanner. it can also be done in some of the new models but I haven't been in the dealer in a couple of years!

Also could be an intake gasket issue. I've seen them crack ad be pulling air from inside the crank case. not always the easest thing to find, but you can try to let the car idle and pull off the oil cap and see if you can hear any sucking! hope this helps. good luck

[torq455]

had a very similar problem in my 97 lumina 3100 when i removed the MAF screen. maybe MAF is going bad? you can find out, i believe, if you unplug the MAF and start the car. if it idles better, it's the MAF. this worked in my dad's 87 IROC-Z, but that was OBD I. so i'm not sure but something to try.

[ssdurango]

adjust TPS

I had a similar problom with my 86 Jimmy 2.8

Take off your TPS and look at it, if it has slotted mounting holes then it is adjustable. If it is then install it loose, and run the engine. Rotate it and rev in up and down until the idle balances out. Give it a try.... let me know if it works for you.