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Old 05-18-2006, 10:58 PM
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Please HELP 98 Vortec engine miss

Stock 1998 Yukon 4wd 5.7L Vortec 177.000 miles business vehicle, dealership maintained

About 2 weeks ago the truck developed a miss. This miss was very intermittent and seemed to happen when hitting a bump or when the transmission shifts. This miss caused no SES light or codes, infact the only indication of a problem at the start was a bucking sensation and the tach diving. This vehicle also had a rough idle at cold start.

I started by replacing the plugs, wires, cap, rotor, fuel filter, and PCV. Still had the miss, so I moved on to an ignition coil, and controll module then distributor. This set p1345. I restabbed the dist,(thanks 1931 Steve for the fax) and read and reset the codes, drove it for about 10 miles, and still the miss but no codes.

I have also ran seafoam through the brake booster, and lucas oil in the fuel and a half tank of premium.

Now with cold start, the rough idle sets p1345 but it will reset and not return. The miss still happens, most often between 1500 and 1700 Rpm.

According to the service records (I recieved all of them when I purchased the truck) The injectors were replaced Nov of 05 with the upgraded ones by the dealership (not sure if I can believe this or not). Also not sure how long this vehicle sat before I purchased it, but I put nearly 2k miles on it both on and off the road before there was any problem. Could this be an injector issue?

I have run out of ideas and could really use some help here. By the time I'm finished I'm gonna need new quarter panels because of the forehead shaped dents

Thanks all, sorry for the length.

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Old 05-19-2006, 06:10 AM
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i've heard of the 5.7 vortecs developing intake leaks between the intake and the head when cold and then sealing up after some heat gets into the engine. I don't know if it's a gasket problem or the bolts loosen over time or what. Do you have a vaccuum guage? i would look for a leak...
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Old 05-19-2006, 07:41 AM
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Check your wire connections and all grounds. Sounds electrical to me.
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Old 05-19-2006, 06:38 PM
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The 1345 code is also intermittent, only seems to be happening when the vehicle has been sitting. It sat over night, and when I started it up this morning, the rough idle was there and it set 1345(twice according to the scanner). I reset it in the parking lot at work, and after sitting 9 hours, I just started it and pulled it inside. No rough idle and no codes.

Also I should mention the miss only happens when driving, not while parked. (left that part out of the initial post)
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Old 05-20-2006, 07:26 AM
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take the upper manifold off. look at the ports. is one really clean ? that a plugged injector. i just replaced a set for this reason, yesterday.
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Old 05-26-2006, 03:21 PM
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progress on this problem

please see the following forum:

http://www.fullsizechevy.com/forums/...d.php?t=211031
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Old 05-26-2006, 07:10 PM
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How old is the fuel?

Maybe theres 2 problems.

Keep checking stuff and eliminating then move on to something else.

If youre 100% sure the distributor is lined up correctly at TDC cyl #1.

Possible that crank position sensor needs replacing.
I read somewhere about them causing false intermitent random misfire code and someting about driving on rough terain can cause it too (must be pretty rough to get that much acceleration and deceleration in drive train though).


The GM service manual is worth its weight in gold for explaining trouble codes as far as Im concerned.



Interesting reading.
http://myweb.accessus.net/~090/dtcobd2p.html#pow

Maybe that will help some.
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Old 05-26-2006, 08:00 PM
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Thanks Greg,

The fuel is only 2 weeks old, and is about 50/50 89 and 92 octane, with 1 bottle of lucas oil. The Dist is a rebuilt unit, so I suppose it could be the sensor.

Guess I could take advantage of the warranty and swap it out as bad, the cap hold down screw holes were stripped anyway (they didnt put the little metal inserts in when they rebuilt it).


http://www.fullsizechevy.com/forums/...d.php?t=211031
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Old 05-26-2006, 08:48 PM
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Did you look at the picture I posted showing rotor position at TDC?

Crank sensor is in front cover. Cam position is in distributor.

I came across a bad cap once that would cause a miss between 2 cylinders cross firing. Probably not your problem though. I threw it away.


Hows fuel pressure?

Good numbers would be.
64 psi with return pinched off

60 psi engine not running

50 psi at idle
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Old 05-26-2006, 10:12 PM
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Fuel pressure

This is new, just came up with a P0339 CKP sensor circuit intermittent code. This came up as a possible, not confirmed code, and did not set the SES light.

Fuel pressures Key on 58# dropping to 52# over 2 minutes, Steady at 54# @ idle

What should the O2 sensors be reading, II'm getting readings anywhere from the 100s to 900s.

http://www.fullsizechevy.com/forums/...=1#post2124387
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Old 05-27-2006, 01:55 AM
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CMP & O2 sensor readings

Just found something interesting while reviewing the autotap data from this afternoons road test. The CMP retard data dropped off the map, and the next frame the O2 sensors across the board dropped into the 50s then the 10s and 20s from the 800's.

I've got another Dist coming tomorrow, since the one I put in was a rebuild and under warranty. Meanwhile, I'm going to Hook her back up to Autotap, and see if I can duplicate the readings by moving the connector and the wiring.

http://www.fullsizechevy.com/forums/...d.php?t=211031

Last edited by Rescurat; 05-27-2006 at 03:20 AM.
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Old 05-27-2006, 03:51 PM
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Progress?

Still waiting for the dist, but attempted to dup the readings in the driveway by wiggling, and pulling on the plugs and wiring for the CKP and CMP sensors, with no luck. So I took her for another ride, and sure enough about 2 miles out she started buckin' and missin'. Sure enough I watched the cmp retard drop out at the same time.

Hope this is it, sounds like it could be a bad sensor. I'll post back when I get the new dist installed. At least I wont make the same mistakes I made with the first one.

http://www.fullsizechevy.com/forums/...77#post2124977
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Old 05-29-2006, 02:47 PM
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Its not the cam or crank position sensors

Well, swapped out the dist for another rebuilt unit yesterday, test drove it today, and still have the problem.

I replaced the CKP sensor this morning, and This also did not fix the problem.

I am considering fuel pump replacement, as I have read that it takes a little while for the bearings to heat up and cause failure usually, (this could explain why the problem cannot be duplicated in the driveway at idle???). Is it possible to check fuel pressure under driving conditions, if so what readings should I be looking for (still 50 to 60 psi)?

I am also concerned about O2 sensors, What should be expected as normal readings? Could a bad O2 sensor cause these problems by telling the computer the wrong mixture?

http://www.fullsizechevy.com/forums/...56#post2127056
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Old 05-31-2006, 12:24 AM
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Just took it for a ride round the block to check fuel pressure while driving. This time it started bucking right out of the blocks, and stalling at low speed (this is new since the new CKP sensor)

The Fuel Pressure remained steady at 54 psi increasing to arround 62 psi with throttle increase, except when the bucking occured, at which time the needle bounced but never really dropped. Also, the fuel guage flipped out, pegged full range full then full range empty, setteling back down (this is also new).

I checked the O2 sensor readings, and were as follows. The upstream (bank 1 sensor 1 and bank 2 sensor 1) read fluctuating readings from the mid 100s to the mid 800s at idle with MAP (manifold absolute pressure) reading 8 to 9 inHg. and the downstream sensors fluctuating in the mid 800s. Then the bank 2 downstream sensor went low around 140came back up into the 800s then back to the 100s every couple of minutes. Average Injector pulses remained between 2.8 and 3.2 ms for both banks.

I pulled the brake booster vacuum line, and the MAP went to 15 in Hg leveling out at 12 with the O2 sensors dropping to the 40s and the injector pulses to about 4.5 ms then level at around 3ms.

When the brake booster was hooked back up MAP went back to 8.6 the injector pulses went back up to around 4.5 ms then back to around 3 ms. All O2 sensors went high, mid 800s to mid 900s Then normalized.

I also checked the CKP harness as best I could, very tight under there. Does anyone know what the plug and pin numbers are for this sensor on the other end of the harness, so I can shoot the wires to check continuity?

The only other thing I did to this thing was degrease the motor to clean it up a bit, so I could change the sensor.
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Old 05-31-2006, 11:40 AM
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Just replacing a sensor due to an intermittant code rarely fixes the issue. You need to do the code diagnosis proceedure. I have attatched the P0339 charts and diagrams for you. HTH,

Freak
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