have a '97 Sunfire GT 2.4/5-speed, 89500 miles, since new. At 1500 - 2000 rpm or so when I get heavy into the gas in 4th or 5th gear I get a stumble or hesitation for a brief moment or two, then once the engine gets a little more rpm (2500) everything is fine. Also from a stop letting the clutch out and ringing it out 1st -whatever the car accelerates fine. If the car is in any gear above say 2500 rpm I don't have an issue. No codes are thrown. I am going to replace the fuel filter anyway, but I believe that since the car is fine under WOT at high rpms, I don't think it is a fuel capacity/pressure issue. I too thought of dirty/old injectors (but I tend to think 90000 is light mileage on injectors), I run Amoco or Mobil 93 since car was new, BTW. I would like to test the relevant sensors, MAP, TPS, EGR??, O2?? Important to note that the car does not do this when cold, so I am convinced it is a sensor that functions when warm (closed loop)
My question is how do you test them, I have a Helm (GM) service manual for the vehicle, and it tells of dianostics on each sensor using a Scan tool set to certain modes, etc, etc. Is there an easier way to test these? Can you use the OBDII scan tool you can borrow from Murray's to run the tests requiring a tool? Am I missing anything?
Any ideas of which sensor is the most likely going bad?
Thanks for your time.
My wild guess is oxygen sensor - but I am no expert. The O2 sensor only operates when your vehicle is warmed up and running off the sensors as opposed to pre-determined fuel tables, and it will also cause stumbling in lower RPM and wouldn't be a factor at high RPM because it wouldn't be included in the rich/lean calculations at WOT.
Your oxygen sensor is a electrical GENERATOR so you can simply unplug it, hook up a voltmeter and read what it reads (once it has been warmed up of course). Low, constant reading or readings that stay below the 400mV level when the engine is rev'd indicate a bad sensor. A simple OBDII scanner may not yet have detected an engine code error but a more complicated one that lets you see sensor inputs will show the above indications as well your "cross counts" will not change. I believe most Haynes manuals contain a simple sensor explanation, but I could be wrong. You surely must be able to find what your looking for by "googling" oxygen sensor.
Sounds like a TPS to me.
Funny story (true story) I had some squirrels under my hood and they chewed some of the wires going to the TPS (didn't cut them). My car did the stumble in any gear at low RPMs, at WOT it was fine.
Jay
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definatly could be the pot. in the TPS, since the engine lives at idle and low RPM's for a lot of it's life, part of it can wear, while the rest is fine.
so did this problem just pop up or has it been this way for a while, i don't know why but i'm thinking valve seals. can't back it up but it's screaming in my head.
Just popped up, in fact the problem is not very noticeable, its only at low rpm in a higher gear. When I say I get a hesitation, it is very slight, I mean you have to pay attention, for example, I know many people who would drive the car and not pickup on it. Also, as I said when cold there is no problem, which made me eliminate the fuel system (injectors, pump, etc) I am 99% convinced it is a sensor problem, just a matter of figuring out which to test first, probably in this order:
1. O2 - hook a voltmeter to it and see what its reading, or if more in depth take it out and put a flame on it, and read it.
2. TPS - Read the values coming off this with engine off - but if it was the TPS, why would it work well cold?
3. EGR - take the vacuum line off and plug the hole, see if it goes away. (Sticking close)
4. MAP - not quite sure how to fully test yet, most likely a voltmeter with engine on, but again, why does it work well cold.
Before you go melting your o2 sensor with a torch please read this;
http://www.forparts.com/o-21.htm
http://www.kemparts.com/TechTalk/tt07.asp
You would see oil come out of the exhaust briefly when you start the vehicle if it was leaking valve seals.
If your Throttle Position Sensor was screwed your computer would not know what gas was being applied, your car would run really bad and your engine light would probably be on. The light probably hasn't come on yet because your cross counts haven't gone that low yet, your catching the problem too early.
So TPS is elimated, right, I figured that too.
What sensors only are used when warm, I know O2. How about MAP sensor? Trying to narrow this down. How about EGR valve?
I probably will test this stuff next week.
"Open Loop" means that your engine is running on a predefined set of variables that determines how it reacts when the gas petal is pushed etc., this conversely means that NO SENSORS are governing its operation.
When the computer has determined that the proper parameters are in place, it is warm enough or whatever it goes to a "Closed Loop" operation, the circuit "closes" and it begins to read from the sensors to determine its fuel delivery etc.
The exact sensors which run in each of the modes of operation:
http://www.speedtweaks.ca/tipsandtricks.php#openclosed
