revving mystery??

Janalee · 06-11-2013, 01:57 PM

Hi all my friends..wish I could have joined the M&G...sigh...
Well..I need some advice...My '99 Chevy 1500 hi-top conversion van has been a wonderful TV. It has the 5.7L eng w factory tow pkg,new radiator, water pump, and brake lines.Yes..I have put a "few " bucks into it but I love this van! It has beautifully towed my 2653 and 2363..and for two seasons, my 30' Wildwood w dinette slide until we downsized to our present Sunny.Well, now she needs an O2 sensor (to turn out the engine light) and there our mystery begins..when she is cold she often rev's in P or N so bad I fear the engine will blow, so you can 't leave her idle. At that time the exhaust fumes are terribly rich. In gear she calms right down and runs smooth.Right now she is getting the O2 installed so they can scan her again..mechanic is a friend of mine and says he really has not seen this before,..first he has to clear that O2 sensor scan to proceed..I won 't be doing my long trips anymore due to our ages and health problems, so when she returns I will hitch up for the rest of the season and park the pair in the side yard w the pressure off the van 's rear end..we will do some shorter trips staying hitched (my poor back!) and decide next Spring what will be.Any of you have some opinions what would cause this extreme revving? Janalee
P.S. There are only 115,000 miles on this van.Serviced regularly.

Gene · 06-11-2013, 05:53 PM

I think quite probably the O2 sensor is what is causing the fast idle when cold. It could take some time with the new O2 sensor for any other codes in the computer to go away and a bad O2 sensor will set off other codes only because it caused the computer to feed too much gas to an engine that didn't need it.

Let us know how it turns out.

mainah · 06-11-2013, 06:58 PM

Until the engine is warned up into closed loop the O2 sensor has no effect on the engine. I would have them check a few things TPS, vacuum leaks and the idle control motor. The smell is more then likely a hot cat from the high idle and not an overly rich mix. There is a pretty good chance that neither of those will set a code so you'll need to find some one with a good scan tool that will read the ECM in real time.

Janalee · 06-12-2013, 08:46 PM

I have posted my thanks to you guys twice but it hasn't shown on here...maybe this time?
So far the O2 is replaced..light is now off. Van stills rev's -luckily did it for the mechanic! Tomorrow they are checking the TPS..and present scan is showing excessive voltage...there is no idle control motor on my van. And he also thinks it is a hot Cat or engine smell not a rich fuel mix. He is mostly concerned about this electrical thing..more tomorrow...Thanks again..Janalee

mainah · 06-13-2013, 05:47 AM

Yeah it does have an idle control device it's called an IAC.

Janalee · 06-13-2013, 08:45 AM

Mainah..I may have misunderstood him about the idle control..this all gets too techie for me..he will be keeping it a few days to check it all out so will follow up. Thanks again...Janalee

beecrazy · 06-22-2013, 09:54 AM

Have they figured it out yet?

I am with mainah, Idle Air Control or vacuum leak.

It can't rev up without air, so it is getting too much air one way or the other. Those engines were notorious for intake gasket leaks (as well as injector spider problems causing misfires)

Good luck and hopefully it won't cost you too much $!!!

Don

Janalee · 06-22-2013, 01:04 PM

Hi guys! Well, since I didn't 't need it quickly they had it for awhile and checked everything over..couldn't 't find anything else wrong. I have had it 3 days and it is really running and idling fine. I think it is running smoother and more zip w the new sensor. They didn't charge for the "searching "..just the O2 and installation...like about $140.00 plus tax. I will be towing this week..hope it is better than ever..I love my old '99 van!! Janalee

Wannabes · 06-22-2013, 01:59 PM

Janalee,

Glad to hear that the revving problem is fixed; sounds like you got a good garage there too. Hope wherever you're towing to, you and your husband have a great time!

dtstanton · 06-23-2013, 09:14 AM

Glad it seems to be fixed.

The O2 sensor can cause the revving by itself. If it detects a rich mixture, it will lean out the fuel mixture and increase the RPMs - mostly noticeable in P or N as you mentioned. Was the sensor "stuck high" (makes sense if the "excessive voltage" was the O2 voltage)? Something else? Was there any spark knock?

Besides the causes already mentioned, a dirty MAF (Mass AirFlow) sensor can do it, too. Drive it for a little while and have him check the LTFT (Long Term Fuel Trim) with the computer if he hasn't thought of it already - just to be sure. If it's high, there is most likely a vacuum leak somewhere, or possibly the MAF.

After replacing any sensor, the van's computer will go through a learning process over so many warmup cycles, so many miles, so much runtime, etc. (depending on the programming). If anything is still a little out of whack, the computer will try to learn what it needs to do to correct a variation before setting another code. That leaning process may very well take care of it as it recalibrates its run parameters. It could come back, but hopefully not.

Still, I would have him recheck the LTFT after using your van (for a week or not long after a week) as I mentioned.

Good luck, and happy camping!
-Dale

mainah · 06-24-2013, 06:31 AM

The O2 sensor is not in the open loop (cold start) and before it does anything it has to be hot (real hot) modern O2 sensors have heaters to make them warm up faster (read emissions) but until then it plays no role in the operation. Fuel management computers do not learn they adapt they have no intelligence. The tables are preset in the ECU it cannot go above or below the preset tables and they are geared for mileage and emissions only. Slight variations in a given sensor can be tailored by the ECU, if it is outside of the given range it cannot "fix" it. A O2 sensor once it is hot adjusts the mix by averaging it’s readings it swings between 0 volts (lean) and 1 volt (rich) as it goes from lean to rich and back again to achieve a proper systolic mix over a given time period. It basically is reading what the other sensor are telling the ECU to do and fine tuning the effort in other words an O2 sensor can’t compensate for a bad temp sensor or TPS. Other than a setting a code the engine will run without noticeable short term effects with the O2 sensor disconnected. Caution should be used in cleaning older hot wire mass air sensor some do not fare well with chemicals and most were self-cleaning they stay on after shut down for a few seconds to burn off contaminates. One of the best ways to trash a mass air sensor is to use an oiled air filter.

dtstanton · 06-24-2013, 11:52 AM

I didn't see where it was in CL. It acts up "when she is cold". The O2 sensors will be hot enough (in the neighborhood of 600 deg. F) and in CL before the coolant temp comes up on the gauge. (I've monitored it many times myself.) I didn't see that it was a "known" that it was in CL, and it's not possible to determine that without a scan tool. It's an unknown in this case (unless I missed something). As for being responsive enough to go CL, the computer checks to see (among other things) how often the O2 output "crosses zero", which is dependent upon sensor temp, not necessarily "when she is cold" temp.

Without regard to semantics, I was trying to explain in a simple/concise non-technical way - since I don't know the technical side of those reading.

dtstanton · 06-24-2013, 01:53 PM

Quote:

Originally Posted by mainah

...an O2 sensor can’t compensate for a bad temp sensor or TPS.

Of course not. The O2 sensor has no processing ability, nor is it in a directly related circuit to them. The computer (not the O2) can and does make tuning adjustments within a certain range, depending on [often] multiple variables - "adapt", if you will. LTFT is an example.

Quote:

Caution should be used in cleaning older hot wire mass air sensor some do not fare well with chemicals and most were self-cleaning they stay on after shut down for a few seconds to burn off contaminates. One of the best ways to trash a mass air sensor is to use an oiled air filter.

Absolutely! There is a proper way to service any component.

Another common problem with "bad sensors" is connection quality. A poor or flaky power/reference/signal/ground connection can cause some strange things to happen in any computer controlled system.

Hopefully, Janalee's problems are cured.