1slow60

I ran the scanner on my truck today while driving to work. With the motor hot all the numbers looked pretty good except for the O2 sensor readings. At a steady cruise it was going from lean to rich which is good but it was going from one extreme to another. It would go from 400 up to 880 back to 400 then down to 79 back back to 400. It was doing this repeately several times a minute. Arnt these swings a little exagerated?? I know that it needs to go back and forth from lean to rich but this is showing way lean and way rich. This was all under a constant light load while I was maintaining 65 MPH on flat ground. My TPS and IAC numbers were constant and this is with a O2 sensor that was replaced about a week ago. Anyone have any feed back on this?? I was trying to see if anythig would possibly cause the rough (but constant) idle that is around 1000RPM. Any help is appreciated guys.

brharris27370

That is what its suppose to do. Look at the BLM to see how the engine is running under cruise.

1slow60

The BLMs are right on at the 124/125 range. I just thought that the O2 readings were a little high in there fluctuation because it was from one extreme to the other. Thanks for the help.

brharris27370

That is a tad rich for a cruise BLM. Have you increased fuel pressure to help upper rpms after mods or something?

1slow60

Nope I havent touched a thing. The motor is compleatly stock and has approx 157,000 miles on her. The only real problems that I am having are a high idle at 1000RPM and an occasional rough idle that only happens when the motor is warm. I dont think the milage is very good though. I am only getting 13.5 MPG with an LO5 5 spd 4x4 with 3.42s. I think that it should be a little higher than this around 16.5 based on a previous truck I had that was similar in setup. I have already done a complete tuneup including the fuel filter so I am at a loss here. I do need to check the timing on the truck as I think it might be a little advanced. I here audible spark knock occasionally. These are just the things that I noticed while driving her. I have only had the truck for the last 800 miles (new to me). Thanks for the help.

brharris27370

Do the coolant temperature numbers look whacked or anything? How many knock counts do you get on lets say a 0-60 full throttle run?

JPrevost

vacuum leak somewhere if the car is idling around 1000rpm. Either that or a stuck IAC but it's doubtful if this is a consitent occurance.

1slow60

The coolant numbers concur with the gauge which appears to be accurate including watching the thermostat cycle. I dont believe that there is a vacuum leak but I need to double check. As far as the IAC things goes it cycles around on its own and the IAC numbers tend to stay below 100 so I am thinking no vacuum leak. My scanner disnt have a area showing knock counts on it but then again I lost the directions to it. If I get a chance I will hook it up again tommorrow and play a little to see if I can find anything. I am going to try and borrow a timing light tonight so I will let ya know what I find. Thanks guys

brharris27370

I would think a vacuum would cause it to run lean instead of rich.

JPrevost

A vac leak can do a lot of crazy things depending on where it is. I've usually noticed the leaks to play havic with the map sensor and so it'll show more pressure in the manifold = more fuel. Anytime something like this happens the o2 volts get all crazy and don't cross count as well with throttle transitions. Only at stead TPS will it cross count like it should. If the IAC counts are up near 100 and it's idling at 1000 then there shouldn't be a leak anywhere. BTW, old o2 sensors tend to take longer to cross count than new.
When I had you're exact problem it turned out to be a bad fuel pump. Darn pressure was changing so frequently with demand that the pump just took it's dandy time to get with the program and my o2 volts would stay at a certain area for a while. Funny thing was that the BLMs didn't change as fast as I would have thought! Really weird.

brharris27370

Sorry Jon, your right. I didn't think about the map sensor. A crack in that vac line would cause some wierdness.

brharris27370

Also, is that 1000 rpm idle according to the tach or the scan tool? I know my tach reads 300 rpm too much at idle and is off by 500 rpm by 3000 rpm.

1slow60

I bought a timing light yesterday but didnt get a chance to use it. My girlfriends car broke. Opps!! O well Is there a way to check the fuel preasure on the pump? I know that the TPI systems have a schrader valve on hte fuel rail but I havent paid that much attention to this systems fuel setup? I need the weekend to get here so that I have a chance to play. Thanks guys I will keep ya posted on what I find.

1slow60

Well I checked the timing today and it is showing to be set at 0 which is the factory setting with the esc 1 wire disconnected. To me it still seems like to much timing in the motor. I usually start out in 2nd gear but recently I notice a little spak knock when i come off the clutch. When I checked the idle speed it was off of the sdcanner cause the truck doesnt have a tach. By the way all the gauges seem to agree withh the scanner. I also checked for vacuum leaks which I found one cracked line going to the charcoal canister in the hard line. This is hooked to an actuator solonoid aint it? That would say that it shouldnt add to the shakey high idle should it? I still need to fix it but I just dont think that this is the problem. What ya'll think? Thanks for the help so far.