Rough Idle on 1998 Silhouette 3.4

[am1971]

I have a 1998 Oldsmobile Silhouette 3.4 with 159,000 miles(highway miles) on the body, but I replaced the motor at 96,000 (due to a blown motor). The current motor recently brought up a code P0172 (System too rich;bank 1). At first it ran fine, but a couple days later I noticed that it started running rough at idle either in park or drive. I also noticed that the longer you left it idling in drive, the worse the idle became (started surging). The OBCII analyzer also showed that #1 cyclinder has a miss. I did the basic things (cheap) and replaced the spark plug and wire to that cylinder. No change... I also had the coil ouput checked, the MAFS and they seemed fine. I even replaced the PVC, aircleaner and fuel filter. When you lightly increase the RPM's in park or drive, the engine smoothes right out. Also at cruising speed there seems to be no loss of power, only at idle, even taking off from a light shows no power loss. I know that it could be a bunch of different things,PCM, leaky injector, etc, but I wondered if anyone had ever experienced this exact thing that might help me to pinpoint it better.

[94 Jimmy]

AM1971
Well you've done it all. How about the O2 sensor? I change mine every time I get into the back of the engine, since it's the only way to get at it and it's such a pain to do.

Recently my van began dying on right turns, like the onramps to the freeway. It finally set a code which said that there were not enough lean/rich transitions in a given time. The computer tracks this sort of thing and if it sees something it dosen't like it lets you know.

My O2 sensor had drifted so that it showed a rich mixture instead of a normal one, the computer reacted to this bad info and leaned out the mixture to the point where the engine would not run under certain conditions. I replaced the sensor and all is good, if you havent changed your O2 sensor in the last 50-60 KMi it's a good bet it's time.

The O2 sensor is essentially a battery that generates a voltage of between .2 and .9V depending on the oxygen in the exhaust. The computer fiddles with the injector on time to keep this voltage between .4 and .6V. It looks for transitions between these two points to indicate that the engine if running in a controlled mode. If it can't force the fuel system to this range it sets a code. As it ages the O2S can get contaminated or just tired and it can give incorrect voltages. Mine read just a little rich but worked in other respects.

I got an OBDII reader from http://www.scantool.net/ it ran about 80 bucks and displays all of your engine info on a PC, Laptop or PDA. It tells you O2 reading, mass air, inlet temp, throttle position, tach ect, while your driving (you drive, let someone else read the guages). It's well worth the money and will make you a hit with your friends.
Hope this helps, let us know what you find
94

[am1971]

Well I pulled the number 1 spark plug out and I noticed coolant on it. I rechecked the oil to see if coolant had managed to enter the oil pan, but I found none. I decided to pull the top off (as I have with so many of the GM v-6 engines) and have a look. The upper and lower intake gaskets were in good shape and no damage noticed. I did however find oil and coolant mixed sludge in the valve covers and just a slight hint in the front head oil valley. The cam area and oil galleys looked absolutly clean and so did the underside of the intake. I figure while I am at it and since I found no real damage to the upper intake gaskets, I might as well tear off the heads and replace the gaskets. I believe I have a bad head gasket or a cracked/warpped head, just from finding the coolant soaked spark plug. I just can't seem to win with GM V6 engines. Every one of them I have had to tear into and replace intake gaskets or others!

[cdru]

Reading your first post, I was going to say that my van started having similar problems at the begining of my head gasket starting to fail. Looks like you figured it out though. You can check in the Venture forum under the "More Overheating" thread I beleive for some tips. Getting down to the heads isn't hard, just time consuming. Hopefully it's just a bad gasket.

When I did mine, the guy behind the parts counter saw that I was getting some odd n ends and asked if I was doing the heads. I said yeah. He immediately responded "Leaking around the #1 cylinder, right?". Apparently that is a troublesome cylinder.

[am1971]

Thanks CDRU. I appreciate the response. That is what makes this a great site to post on. It's nice that you can post something and wait to see if someone repsponds with the same problem. That just helps pinpoint the problem.

[am1971]

cdru, pulled the heads off last night. Found the head gasket cracked at cylinder #1. All others looked fine!. I will be taking them into a local shop to have them pressure tested and have the surface refinished, then re-installing them. Piston #1 was "steam cleaned" from the coolant! I new immediately when I pulled the head that was the problem. I have heard they use inferior gasket material. I have torn into many v-6 engines(I worked at a shop that built race motors), and replaced head and intake gaskets, but by far this is the most radiculous set-up I have ever seen! I cannot believe the amount of items that have to be removed to do this job and then on top of things I had to beat the upper engine mount (located on the core support) off of the pin on the front head, just so I could get at the head bolt which is the pin for the mount. What a joke! Thanks for all who input, that is what makes this site so successful, we can all profit from our mistakes and past problems and keep our cars out of the dealerships! Keep up the good work.

[cdru]

In GM's defense, where are you going to put all the stuff? In a car, you have a lot more room so you don't have to stack everything on top of each other. But with the engine compartment in our vans, you gotta stack stuff. Astro/Safari's had (and maybe still do) the advantage of the engine compartment that stuck into the cabin. GM decided to sacrifice that engine compartment space for cabin space.

It is rediculous the amount of stuff that you have to (re)move, but I'll take my van head over my old Talon's head any day.

[dworakow]

Jimmy,
I have a similar problem with my Venture2000. 90000 miles, no check engine warning light, very high fuel consumption in a city driving (10-12 mpg). I was advised to change the sensors. I m not a mechanic, and this is my first car. Do you need a lot of experience to change the O2 sensor located before converter?
Can I change it using ordinary tools?
And the last question: Is it a good idea to buy the sensors online? Local dealer charges twice more for the part.
Will be grateful for any tips
Dworakow


Looks like I have to change the O2 sensors.

Originally Posted by 94 Jimmy

AM1971
Well you've done it all. How about the O2 sensor? I change mine every time I get into the back of the engine, since it's the only way to get at it and it's such a pain to do.

Recently my van began dying on right turns, like the onramps to the freeway. It finally set a code which said that there were not enough lean/rich transitions in a given time. The computer tracks this sort of thing and if it sees something it dosen't like it lets you know.

My O2 sensor had drifted so that it showed a rich mixture instead of a normal one, the computer reacted to this bad info and leaned out the mixture to the point where the engine would not run under certain conditions. I replaced the sensor and all is good, if you havent changed your O2 sensor in the last 50-60 KMi it's a good bet it's time.

The O2 sensor is essentially a battery that generates a voltage of between .2 and .9V depending on the oxygen in the exhaust. The computer fiddles with the injector on time to keep this voltage between .4 and .6V. It looks for transitions between these two points to indicate that the engine if running in a controlled mode. If it can't force the fuel system to this range it sets a code. As it ages the O2S can get contaminated or just tired and it can give incorrect voltages. Mine read just a little rich but worked in other respects.

I got an OBDII reader from http://www.scantool.net/ it ran about 80 bucks and displays all of your engine info on a PC, Laptop or PDA. It tells you O2 reading, mass air, inlet temp, throttle position, tach ect, while your driving (you drive, let someone else read the guages). It's well worth the money and will make you a hit with your friends.
Hope this helps, let us know what you find
94

[cdru]

Originally Posted by dworakow

Do you need a lot of experience to change the O2 sensor located before converter?
Can I change it using ordinary tools?
And the last question: Is it a good idea to buy the sensors online? Local dealer charges twice more for the part.
Will be grateful for any tips

The upstream O2 sensor isn't in the easiest to get to place. I'm presuming that you don't want to tear apart half the engine to get to it. You just want to make it as easy as possible, right?

You'll need to jack up one side of the vehicle, both front and back. USE SAFETY JACKS. You need to get access to the exhaust pipe from underneith the vehicle. You can reach the O2 sensor from the bottom reaching up in the gap between the floorpan/firewall and the exhaust pipe. It's the only thing sticking up out of the exhaust pipe between the cat and the engine, where the individual cylinder exhaust ports come together to bolt up to the exhaust pipe.

A special slotted socket is all but required to get the old sensor off and the new sensor on due to the wire pigtale. You can get the socket at most autopart stores. Some, like autozone will let you borrow it for the purchase price and then refund it entirely when you return it. I'm not sure if there is enough clearance to get it off with a slotted wrench easily. Rolling the engine forward may be required for easier access and clearance...I don't remember from when I did mine a year or two ago.

Installing the new one is just the opposite of reverse. Make sure there is antisieze on the threads (most come with it already). Make sure you don't damage the wires when reinstalling it as well as don't touch/damage the business end of the sensor. It is very sensitive to damage as well as chemicals.

You can buy online without issues if you stick with name brand. I'd just go down to your local autoparts store. I know of no reason why OEM would be better or worse then aftermarket. Either way should come with limited warranty.

[am1971]

Well, problem solved. Tore off the top half of the engine including the heads and discovered that cylinder#1 had in deed cracked a gasket. Also with it, the heads showed hairline cracks (1 on each head) jumping across from the intake valve to the spark plug threads. Per the machine shop, they had never seen this problem on this motor before. Anyway, had another set laying around and they too had small cracks no where near as bad). Well they machined them and pressure tested them and they checked out okay. I am assuming that this is more common then thought to be, considering my engine only had 45,000 miles on it. I replaced all gaskets and a few other things and it is running excellent now. I am actually having some fun with it since the heads were milled and refinished , seems to have increased the compression ratio enough to give it an extra bit of go! From what I understand, this is a common problem with this motor. So if you notice your coolant level keeps dropping, engine is idling rough and the oil shows clean on the dipstick....you might want to concider the heads as the culprit!! Thanks for the help!