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slight hesitation on acceleration
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milling machine
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Jun 5, 2010, 3:58 PM
Post #1 of 11
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slight hesitation on acceleration
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I have a 1994 toyota pick-up 4cyl. manual trans 22re engine. it has a slight hesitation on acceleration that I traced to a bad vaf sensor I have a analog scope and I can see slight opens in the trace, I am certain this is the problem, but I was wondering when you spray carb cleaner on the fuel injector o-rings and your oxygen sensor immediately goes rich and the tachometer jumps about 5 to 10 rpms is this enough to cause a lean missfire because spraying all my fuel injector o-rings causes the O2 sensor to go immediately rich and pick-up about 5 to 10 rpms and so does spraying the throttle body shaft. But my question is how much of a rpm increase would be enough to consider that there is a big enough vacuum leak to cause a miss-fire ?
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Hammer Time
Ultimate Carjunky
/ Moderator
Jun 5, 2010, 4:28 PM
Post #2 of 11
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Re: slight hesitation on acceleration
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Any RPM increase at all is a sign of a unacceptable vacuum leak. You can't even start to look for a miss if you have known vacuum leaks. You have to resolve all of those and then see what you have then.
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milling machine
User
Jun 5, 2010, 5:07 PM
Post #3 of 11
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Re: slight hesitation on acceleration
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Thank-you, I did not know that
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Sidom
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Jun 5, 2010, 9:27 PM
Post #4 of 11
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Re: slight hesitation on acceleration
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Like HT said, you really need to make sure there aren't any vacuum leaks 1st. If you have any propane, you can run some of that into the throttle body while it's idling, if it smooths out you have a vacuum leak for sure...
I'm curious about what you mean by "opens" in the pattern. That should range from around 2 to 7 or 8 volts depending on the air door, It should be a smooth transition with no glitches {spikes} in the pattern but there shouldn't be any open spots in the trace. You could also scope the signal wire on the O2 make sure everything there is good since you do have a scope handy. It should be switching between 100mv & 8 to 900mvs.
Those old analog scopes are great. I love the pattern, they're so sharp & clear, way better than any DSO I've seen or used. Unfortunately you can't freeze, zoom, review, store, invert, use cursors, etc with an analog 
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milling machine
User
Jun 7, 2010, 4:56 PM
Post #6 of 11
(5280 views)
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Re: slight hesitation on acceleration
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The vaf basically has a normal scope pattern like when you read a normal maf on a scope except it has small opens in some parts of the trace that I think are causing it to run lean (but not certain). Also, it has what I call a lazy sensor because it will not start to work until you give it a burst of throttle body cleaner then it is normal, I can see it on the scope it also has normal transition times. What I do is take a video of the scope trace while the car is running with my cell phone then you can play it back and even stop the video and examine the trace pattern or even put in on your computer and examine it there. The worst thing about this vehicle is you cannot use a scanner. Thanks for the answers
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milling machine
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Jun 14, 2010, 8:03 PM
Post #9 of 11
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Re: slight hesitation on acceleration
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I could not answer back my computer was not working for the last couple of days but you say the 02 sensor is reporting lean and correctly but when I squirt it with throttle body cleaner it starts to react normally going from lean to rich. And I do not know how to post the traces on the computer I just look at them on the cell phone. Anyway, I will not be working on this truck till the end of summer to hot here in Las Vegas, everyday here is over 100 degrees
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Sidom
Veteran
/ Moderator
Jun 14, 2010, 10:49 PM
Post #10 of 11
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Re: slight hesitation on acceleration
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That tends to support HTs vacuum leak theory. With a leak, the O2 will report lean just for the fact with the leak, the air/fuel mix is lean.
Squirting throttle body cleaner is similar to propane enrichment. The purpose of that test is to see if the O2 is capable of switching or not and in the proper time frame. When enriching if the O2 doesn't switch or can't switch from .075mv to .850mv in 100ms after the enrichment is removed then the sensor is bad and could be the cause. If it does switch and within the time division the sensor is good and some other problem is causing it stay lean (or stay stuck rich in overfueling cases).
It was a basic test but from the results, sounds like O2 are good & the problem is some other place.
Yea we're finally get our heat as well, it's been very late in coming but triple digits are finally here...........
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milling machine
User
Jun 15, 2010, 7:47 AM
Post #11 of 11
(5235 views)
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Re: slight hesitation on acceleration
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Thanks for the response
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