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1996 gmc k1500 5.7 vortec

  • Cberry121
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04 Sep 2019 05:39 #33404 by Cberry121
1996 gmc k1500 5.7 vortec was created by Cberry121
Ok guys, been banging my head against the wall on this one. Truck was running great, went down with a leaking intake manifold. Here's what I did, replaced head gaskets, all manifold gaskets and o rings, put a new distributor on as the old one was cracked. Egr delete kit, new plugs and wires. Adjusted valves.
Truck now has a random but pretty persistent misfire that can be seen on all cylinders with a scanner, doesnt donit when idling.
Tests i have done..1. fuel pressure Is at 55 psi running, doesnt hold with key off though. Pulled upper intake and see no fuel.leaking by regulator or inectors.
2. Vacuum test was 20 in hg steady at idle. Perfect
3. All cylinders 160 psi +/- 2 compression
4. Maf only reading 1.0 g/s on scanner
5. Map only reading 9.4 inhg on scanner
6. O2 sensor bank 2 sensor 2 set code for high voltage. Reading 1.1v
7. Smoke tested for vacuum leak and found nothing
8. Cmp retard is at 1.0° (within spec)
9. Timing advance is roughly at -33

Only other code it shows is insufficient egr, which inhave ordered a new tube, but not sure how that could relate to low MAF and MAP readings.

The misfires themselves dont even create a check engine but i can feel them and count them on my scanner

Not sure where else to look, any ideas?

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04 Sep 2019 09:44 - 04 Sep 2019 09:47 #33407 by John Curtis
Replied by John Curtis on topic 1996 gmc k1500 5.7 vortec
What does the upstream 02 ready? Does upstream and downstream agree? The high voltage is very likely from the EGR delete and I would think you’re best bet would be to take it to a tuner.

MAF is WAAAAAY low. Disconnect it just and see what happens. Default strategy might be better than an under reporting MAF.

MAP is also low.

I know you said you checked for vacuum leaks but this makes me lean toward a vacuum leak. I don’t think it would be an air intake restriction from your vacuum gauge readings. Have you checked PCV?

Thinking out loud always helps me in the process.
Last edit: 04 Sep 2019 09:47 by John Curtis.

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04 Sep 2019 10:44 #33408 by ontheriver
Replied by ontheriver on topic 1996 gmc k1500 5.7 vortec
How old is the spider? another item,you are showing data at idle,when the problem is at higher rpm,need to test in that rpm range

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04 Sep 2019 11:30 #33409 by Cberry121
Replied by Cberry121 on topic 1996 gmc k1500 5.7 vortec
I did do a smoke test for a vacuum leak, and didn't find anything. I do however hear something hissing around where the pcv connects to Intake. But I've sprayed it and everything and i cant find it still..

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04 Sep 2019 11:33 - 04 Sep 2019 11:35 #33410 by Cberry121
Replied by Cberry121 on topic 1996 gmc k1500 5.7 vortec
The MAF AND MAP readings dont change much above an idle, the maf goes to 2 g/s max which is will low, cmp readings were done at 1000 rpm. O2 sensor voltage goes up with rpm as well. Not sure how old the spider is, but it doesnt leak at the regulator or the injectors. Tested that with the upper intake off and cycling power on
Last edit: 04 Sep 2019 11:35 by Cberry121.

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04 Sep 2019 11:35 #33411 by ontheriver
Replied by ontheriver on topic 1996 gmc k1500 5.7 vortec
Do as mentioned above, disconnect MAF with key off,run it,any difference?,and what are the front 02's reading at problem rpm,wild?

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04 Sep 2019 11:41 #33412 by ontheriver
Replied by ontheriver on topic 1996 gmc k1500 5.7 vortec
Clogged cat?MAF will read low if exhaust is restricted,rule of thumb,MAF,GPS should read the same as litres of motor at idle,so 5ish warm

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04 Sep 2019 11:48 #33413 by ontheriver
Replied by ontheriver on topic 1996 gmc k1500 5.7 vortec
One problem with the poppets in spider,the older they get,clogged,bad spray pattern,the fuel pressure needs to be on the high side,domino effect,are the front 02's switching wildly?and what bank is worse

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04 Sep 2019 18:41 #33418 by Vw mike
Replied by Vw mike on topic 1996 gmc k1500 5.7 vortec
It has been few years but I remember a cam crank relearn on these.

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20 Sep 2019 12:14 #33722 by Cberry121
Replied by Cberry121 on topic 1996 gmc k1500 5.7 vortec
So after having the upper plenum off to check for spider fuel leaks, I re installed everything, didn't change any parts and now my misfire is in cylinders 4 and 8 only, instead of a few misfores across all cylinders

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