Chevy High Performance
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jr85 jr85
New User | Posts: 4 | Joined: 12/12
Posted: 01/12/13
08:25 AM

hi guys got a ? that hopefully someone can answer i have a 98 chevy 2500 service body vortec 5.7 i was driving home and it started spark knocking so bad that i thought the motor had blown. took it to a shop new distributor and computer and all that did was waste $300 bucks runs exactly the same the shop said that as soon as it cranks up it is advancing the timing all the way. i really need this truck fixed cause as long as its home from work i am to so any ideas are greatly appreciated.  

waynep71222 waynep71222
Enthusiast | Posts: 448 | Joined: 03/12
Posted: 01/12/13
10:16 AM

i had that EXACT issue on a 98 4.3 astro van...

this is going to require an ADVANCED scan tool that can control the various systems..

get into the OEM tests.. find the EGR testing section...

it has a digital EGR valve... with the engine off.. key on..  ramp the EGR valve open and closed through 10% then 20%....  then 30%...  all the way to 100% and then closed.. several times..

i normally do this.. but i had training and experience... with the engine running...  keeping it running when this happens with additional throttle input...

when i ran across this issue.. the EGR could be opened to 50% without effecting idle..  it should have died at 10 or 20 %.

if you take the truck for a drive and monitor the EGR request.. you will see that the EGR opens to around 45% at 35 to 38 MPH..

i use a newer version of the OTC 4000E.. called the monitor extended to ramp the EGR open and closed.. most professional scan tools will do this with the proper program cartridge and cable..  consumer units usually will NOT do this...


the snap on MT2500 will also do this with the proper cables and cartridges..


as will other models like the genisys


what happens is the computer counts on the EGR flow to dilute the oxygen content in the incoming air charge.... this reduces the oxygen content..   less oxygen ... less fuel needed to be sprayed in...  so without EGR flow. but with the valve seeming to operate... the engine will run lean and ping like crazy..

you can just replace the EGR valve... but i would use a wooden stick if you can.. and block the intake side of the passage... while somebody starts the engine with the EGR valve off...  so it blows any carbon out of the passage.. .  please use eye protection when doing this..  or look away.. sometimes carbon comes out fairly fast..

it also depends on where the EGR Valve is.. on the 4.3s its right up front.. i don't recall where it is on the V8 motors..  

pepsi1 pepsi1
Guru | Posts: 1718 | Joined: 09/11
Posted: 01/12/13
10:57 AM

Did they get any knock sensor codes?


waynep71222 waynep71222
Enthusiast | Posts: 448 | Joined: 03/12
Posted: 01/12/13
01:08 PM

you are correct on that...

there are a few ways to check that if they don't have a scan tool.. to access codes..

with a timing light hooked up to the #1 spark plug wire.. somebody sitting in the drivers seat...  parking brake applied...    if it has an automatic transmission...   FOOT ON THE MAIN BRAKE PEDAL... truck in drive...     while watching the harmonic damper marks with the flashing light.. have another friend tap on the engine block or cylinder head with a LONG screwdriver...  or a long extension..  the timing should retard... as soon as the helper knocks on the block.. this is visible with scan tools....

DO NOT on these VORTEC motors.. attempt to adjust the ignition timing by turning the distributer... all it will do is mess up the fuel injection... these motors have CRANK SENSORS plugged into the 7 o clock position on the timing cover..  the device in the distributer is the cam shaft position sensor..   if the distributer drive gear is failing.. and it does happen.. or the housing bushing is failing... the distributer might need to be replaced..

before one does that.. they can carefully remove the distributer cap...  check the underside... for signs of burn thru.. there can be hidden cross arcing between the conductors running back and forth in the cap... as i have outlined below..


check the rotor... for signs of excessive carbon..  grab the rotor.. attempt to twist it and the shaft back and forth.. there should be no play in the distributer shaft to cam gear... you should not be able to feel more than a TINY TINY fraction of a turn. if any..

lastly for now..

there is one more thing...  you might want to clean the bottom of the timing cover where the crank sensor is installed... remove the crank sensor... examine the end..  where the reluctor on the crank snout passes the end of the crank sensor.. there have been instances where something got screwy and the crank sensor tip is being impacted by the rotating reluctor /tone wheel..  this really screws with it.. there is a TSB on this... and special shims to move it slightly farther back from the reluctor...  

waynep71222 waynep71222
Enthusiast | Posts: 448 | Joined: 03/12
Posted: 01/12/13
01:18 PM

there is another way to tell if the cap has internal cross firing going on..

use your timing light... and hook one at a time to individual spark plug wires...  you will be able to see if you get an oddly timed double flash instead of the single flashes..

look how close the Coil and #6 are...    the 5 and 3.... the 3 and 2 and 4 in 2 different locations..

the 7 and 2...  the 2 and 8....  the 3 and the coil are also very close..

one will also want to check that all the spark plug wires are pushed on properly and that there is no corrosion build up in the cap to wire connectors... and that the terminals inside the spark plug end of the wires are not pulled off..

clogged injector nozzles may also cause knocking.. as its leaning out one cylinder...

there is an updated injector set.. that instead of having a single injector to control all the nozzles.. its has 8 individual injectors at the ends of the hoses...  

jr85 jr85
New User | Posts: 4 | Joined: 12/12
Posted: 01/12/13
08:08 PM

i don think it the cap it has been changed serval times and runs the same everytime no knock sensor codes but the shop claims the knock sensor couldnt cause what its doing im not to sure about that  

waynep71222 waynep71222
Enthusiast | Posts: 448 | Joined: 03/12
Posted: 01/12/13
11:58 PM

few people will think that a STUCK or clogged EGR valve will make the ENGINE PING... but it will

ask them if they know how to use the scan tool to open and close the EGR valve a bunch of times to get it working again..

take it for a test drive with the passenger looking at the EGR percentages while moving along...

i got the EGR open with a scan tool to 40% before the engine even stumbled  at idle.. indicating the EGR was NOT responding to ECM input..  it was a TOTALLY DIFFERENT truck once  i had operated the EGR a few times.... no pinging.. better fuel economy... more power..  

redneckjoe69 redneckjoe69
Addict | Posts: 2337 | Joined: 03/10
Posted: 01/13/13
03:23 AM

out of curiousity,...did it start running bad after getting gas?     maybe you got some krap gas?