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1998 chevy 3500 5.7 liter p0507

posted May 05, 2016 23:06:35 by cheryl hartkorn
having issues with high idle. shop replaced iac valve. looking at scan data seen the engine coolant temp sensor was reading 30 degrees at operating temperature. replaced that now it goes into closed loop. looking at fuel trims bank 2 is very negative. -30 short term so if it was a vacuum leak id expect to positive numbers. also iac count is at 0. idles at 1300 rpm desired 700. got any ideas?
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10 replies
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Tyler said May 06, 2016 02:04:11
Yo Cheryl!

So bank two short terms are pegged out, how is bank one doing? Pegging the trims out on one bank says failed O2 to me, maybe time for an O2 response test. Of course, both banks pegging rich would scream leaking fuel pressure regulator, but I feel like you would have mentioned that...

Despite the fuel trims, I still think you have some kind of vacuum leak causing the high idle. Pull/plug all the vacuum and PCV hoses and see if the idle gets under control. If not, remove the intake tube and block the throttle body completely. The engine SHOULD stall. If it doesn't, there's still a leak.
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AndyMacFadyen said May 06, 2016 07:58:53
Tyler hits it on the nose.
High idle speed implies engine must be getting extra air from somewhere. Sometimes you can find a vacuum leak with a stephoscope.
What are the long terms trims like ? I wonder if the trims could be stuck a battery disconnect might reset them.
"Rust never sleeps"
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cheryl hartkorn said May 06, 2016 21:58:51
tested the o2 sensors. they react. I tried the covering the throttle body with my hand it. didn't stall forgot to mention that. it made a loud whistling noise while I had it covered. pcv?
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Chris said May 07, 2016 01:22:21
Id start looking at the Intake gaskets. Below the throttle body, upper plenum and lower manifold. The factory lower manifold gaskets are prone to cracking (usually mimick a failed head gasket when they do). Check around the injector pass thru connector. Seen guys F that O ring up after replacing the FPR and injector spider.

This is screaming for the water bottle test and smoke machine. As far as the negative trim on B2 check your injector waveforms and see if one is grounded all the time. I'm betting there's a dual issue here
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Tyler said May 07, 2016 13:51:33
I tried the covering the throttle body with my hand it. didn't stall


Nice! Yeah, Chris is right on, water test all the way. Cover the throttle with a phone book or something, then go to town with the spray bottle.
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cheryl hartkorn said May 07, 2016 16:52:09
so does covering the throttle body amplify a vacuum leak? for example your cutting off the air supply to the engine through the throttle body and cause the vacuum leak in question to suck more air in to keep rhe motor running? is that why you say cover the throttle body with something then look for a leak with the water bottle? better than a smoke machine?
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Tyler said May 07, 2016 17:02:45
so does covering the throttle body amplify a vacuum leak? for example your cutting off the air supply to the engine through the throttle body and cause the vacuum leak in question to suck more air in to keep rhe motor running?


Huh, never thought of it that way! For me, covering the throttle body is mostly about proving the existence of a vacuum leak. But, it probably does force the leak to suck more air, and therefore make it easier to find.

I threw the phone book thing in just so you could free up your hand for leak checking, rather than have one getting sucked into the throttle.
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Noah said May 09, 2016 00:19:25
Cover the throttle with a phone book

Yeah, second drawer from the bottom on the right, with the other obsolete publications. Lol!

Just joshing you, definately a valid test, just struck me funny.
Massachusetts, USA
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Tyler said May 09, 2016 01:53:57
Hah! Yeah, you got me there. The phone books that (still) come to our house go straight to the round file.

I think I had phone books on my mind because I was talking to a diesel tech who keeps one handy. Apparently they're good to have around in case an engine decides to run away during service O_O
[Last edited May 09, 2016 01:55:15]
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Chris said May 09, 2016 16:30:27
Run away diesels are no joke. Seen 2 in my career. One lived thru it the other evacuated its internals.
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