TBI still missing, failed emissions, code 34...

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rob249

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Still trying to get my 94' Yukon to stop missing at idle. I did a tune up before a long trip a few months ago, new plugs, wires, coil, distributor, and "rebuilt" the tbi. Truck was running great, except for the miss at idle when warm. I failed emissions, high HC, but the CO is quite low. An old GM mechanic suggested I check the EGR, which did have a leaky diaphragm. I replaced that, now Im getting a check engine light, code 34, which is MAP sensor high vacuum. Wondering if i have the vacuum line going to the wrong place, or if its just a bad MAP. Vac line is going from MAP to the back of the TBI. No codes before the new EGR. Any way to test the MAP? Should I just assume that sensor is bad? Maybe I have the vac lines incorrect? Please advise, and thank you.
 

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rob249

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I did not, but I would assume it would send a code 53 or 55 for that, but you know what they say about assumptions.....

edit; Dont know if this is relevant, but a few years ago, I had a no start situation. It was a bad MAP, my buddy had a used one that came out of a van that we replaced it with. Perhaps the van MAPs are different?
 
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GoToGuy

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Do you still have vacuum line routing decal stuck on the radiator top support under hood? If not look in service manual for line routing. If you don't have it, get it.
It's here in the forum for free download. A Lot of the easy questions can be answered for you.
 

rob249

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I appear to have the Vac lines correct as per the diagram, thanks for the info, I don't have the sticker, although i can see where it was.
 

heskahoka

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Check and recheck your vacuum line for cracks/leaks from the MAP the to throttle body. I like to use zip ties on mine just to be sure.
Also re-check the vacuum lines elsewhere since the Code 34 popped up after you changed the EGR diaphram.

As GoToGuy said, check out the 1994 Emission link here:
There is a procedure for Code 34 and checking the MAP.

Good luck.
 

rob249

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Due to the unknown age of my MAP sensor, I opted to just replace the thing. I double checked the VAC line, and broke the last OEM hard plastic line, so now, all of my VAC lines have been replaced with tight fitting hose. The new MAP got rid of the code 34, but, I still have a rough idle, probably wont pass emissions still. Back to the drawing board I suppose, thanks for the info everyone, this is definitely the best forum for these trucks. Ill update if and when I figure it out.
 

L31MaxExpress

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Due to the unknown age of my MAP sensor, I opted to just replace the thing. I double checked the VAC line, and broke the last OEM hard plastic line, so now, all of my VAC lines have been replaced with tight fitting hose. The new MAP got rid of the code 34, but, I still have a rough idle, probably wont pass emissions still. Back to the drawing board I suppose, thanks for the info everyone, this is definitely the best forum for these trucks. Ill update if and when I figure it out.

TBI base gasket or intake gaskets can both fail and leak causing a vacuum leak and a rough idle.
 

rob249

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I rebuilt the TBI a few months ago, new o-rings, gaskets, etc. This included the TBI base gasket, didn't mess with the intake yet, that's a bigger job i was hoping to avoid. The miss was only occasional until recently. I am currently talking to my buddies dad, he is a retired GM mechanic, he'll have some ideas.
 
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