Here's one for the experimenters and GM philosophers

[89sportside]

Okay so!

I'm having an issue with my 1989 K1500 350. A few weeks ago I was getting rich fuel, no start (flooding) problems. I did some tweaking and got that fixed, and after that the off-idle stumble surfaced. After some more tweaking I found that plugging the vacuum hose at the EGR valve eliminated the stumble, but I soon noticed I had a knocking sound under a load.

Basically, with the EGR plugged in it will die just off idle (backing out of a parking spot, pulling off from a stoplight) and no SES light. and with the EGR plugged I get a knock in my engine and sometimes SEE light. Octane Booster helps but doesnt eliminate the knock. What I want to find out is which component is failing me.

Here's what I've already done:
New cap, rotor, plugs, wires.
New coolant temp sensor
New MAP sensor
TPS is known to be good
Checked PCV valve and hose operation
Replaced all hose between intake, solenoid, and EGR
Replaced vacuum line to charcoal can
Set ignition timing to 0*
New fuel filter
New air filter

I also pulled out the EGR valve and by applying positive and negative pressure to the vacuum inlet I can hear the diaphragm (poppet?) move in and out. Also, by applying battery voltage to the solenoid I can hear it open and close accordingly. The vacuum routes between the vent and inlet correctly when I do this.

My theory is as follows:

Either a) my EGR valve isn't closing all the way (carbon buildup? I sprayed an obscene amount of carb cleaner through it just in case) which is allowing exhaust to seep in at low rpm

b) the solenoid is opening earlier than it should, which allows exhaust to seep in at low rpm

Or C) my exhaust gas has the incorrect chemical makeup, causing the fuel to fail to ignite (which seems unlikely)

I don't care whether the EGR stays part of the intake or not. I'm hoping someone can either help me diagnose and fix the problem with the EGR, or figure out a way to lower my engine temperature without it so as to cure the knock. Retarding the timing is a last resort since its at manufacturer specifications (which is 0* TDC) now.

I know someone here will be able to help with this. There are some sharp people on this forum. Thank you all

 

[Manimal]

Hows the spray pattern on you injectors? Is it a spray, mist or dribble? Whats your fuel pressure? If you starve it for fuel and introduce exhaust from EGR, it will die or stumble badly.

 

[89sportside]

Injector spray pattern is pretty normal, no dribble and keeps a steady pattern throughout throttle range. I checked it with a timing light on my distributor wire.

Fuel pressure appears to be okay but I haven't checked because NAPA wants a 150 deposit for me to rent a gauge.

When the EGR vacuum inlet is blocked off I get good throttle response, good everything until the engine warms up and starts knocking. Do you think I could have low pressure even in conjunction with that? It just doesn't look or act like that's the case unless the egr is hooked up.

 

[89sportside]

Of course, it's possible that low fuel pressure could cause it to run lean, which creates stumble when exhaust gas is introduced, and causes detonation when exhaust gas isn't available?

 

[Manimal]

Bad gas can also be a culprit. low octane will cause spark knock, high cylinder pressure/temperature and not enough octane will cause spark knock. Timing advanced will cause spark knock. EGR is designed to cool the combustion chamber and reduce NOx. Before you go too much further, make sure youre TDC on COMPRESSION stroke and verify the balancer is on the mark. small block are notorious for the balancer ring slipping. If it has and you have set the timing by the balancer, you may be WAY off.

 

[Manimal]

When I find TDC, I always remove all 8 plugs(just makes it easier to rotate by hand), remove distributor cap AND driver side valve cover. I rotate by hand until the #1 intake valve is about to close, then I use a screw driver in the plug hole and continue to rotate slowly until the piston is at the VERY TOP. HOLD the screwdriver in your hand and youll feel the piston when it reaches TDC and if it begins to rock to go back down..you want it at the VERY TOP. Thats when youve found TDC on COMPRESSION stroke. Then verify your distributor/rotor location to #1 on your cap, and verify the mark on the balancer to the timing tab on the front cover.......ALSO check to make sure you have the correct firing order.(it happens to us all)

 

[90halfton]

You can buy a fuel psi tester and Schrader valve doohickey from actron for far less than the rental fee you mentioned

 

[OlSmokie]

I recently replaced my exhaust manifolds, and when we went to take the O2 sensor out, the wire came right out of it. I had to put a new one in and my truck has not run this good in years, (1990 C1500). I had no lights or stored codes, but I was having all kinds of issues with starting and stumbling even after replacing cap,rotor,plugs,etc. Also, you did unplug the wire under the hood when you set the timing, right?

 

[Blackwater]

That's a good point. I suggest rebuilding the regulator in the TB if it hasn't been done in a while. I have don it once. Also, you may have to change the knock sensor on the right side of the block and check to see if the exhaust hasn't burned or damage to the wire to the sensor.

 

[J_lope82]

When you say knock do you mean an actual knock or tapping mechanical noise on the engine or do you mean detonation/pre-ignition/pingin/spark knock?
What makes you think the engine was flooding out and what did you do to fix it?