Timing gets thrown off

[Lt.Del]

OK guys, here's one for ya...my 1991 3/4 ton 5.7 litre 4x4 suburban is not running right.

Something is throwing the timing off, like advancing way too much or retarding way too much.  I have replaced the coil and cap/rotor.  No change.

She will run well for about 2 miles, then she acts up....like it's running on only two cylinders...it wont accelerate and I assume the timing gets thrown way off. It wont go faster unless i am going downhill.  It backfires some if i really floor it--the horsepower gets reduced to about 10hp. After its warm, it even does this at idle...I can tell she doesn't sound right....when i punch the accelerator, she just takes time to climb rpms.  It's like it is coughing sick.

So, I figure it may be the knock sensor telling the ESC (electronic spark control) to adjust the volts going to the ECM, thinking it is knocking or something.  I really believe this problem has to do with the knock sensor or ESC.  If i replace the sensor and ESC, well, it may then be the ECM if it persists.  Problem is, i dont know where any of these babies are.  Please look at the pics. 

Is pic C the knock sensor?  Is pic B the ESC?  What is on the black bracket in pic A?? I guess the ECM is under my glove box inside. Right??

I don't want to turn this sub into a shop and them charge me $500 to tell me they replaced these parts.  If these parts are less than $200, well, i will have saved money. I know its a gamble, but I like to figure things out on my own.  It could well be a connection or wire grounding as well.  I am willing to take the risk by replacing the parts and see what happens.  It really seems as if the timing belt has jumped, but, it runs well for about two miles when cold.  The service engine light does not come on after initial startup.

Any ideas???

[77c15]

Del...pic C is the coolant temp sensor...the knock sensor should be on the side of the block, passenger side if I remember right on those newer small blocks, right below the back spark plug but above the oil filter and/or starter. Should look fairly wierd, like a huge sensor.

[Lt.Del]

Thanks--I'll look there. 
If that's the case, what is the sensor located right behind the number one spark plug with the green wire?

According to the books I am looking at, the knock sensor should have only one wire.

Pic from Advanced autoparts web site. 

[okiesinmt]

Del-pic A is to dark to tell what it is,  pic b looks like the MAP sensor, and pic C is the coolant sensor.  Looks like a small cap distributor in the photo, if that is the case it sounds like a cracked reluctor which is in the distributor.  The reason it runs alright from cold is because the timing reference is coming from the control module,once it warms up a little the timing signal goes to the ECM from the reluctor.  I suggest switching to a new distributor, Plenty of aftermarket, MSD is my choice, but a little spendy.

[VileZambonie]

A is your EGR solenoid B is your Map sensor and C is your coolant temp sensor.

The ESC module is designed to retard the timing when a knock signal is generated from the knock sensor which is a piezoelectric crystal. The louder the knock generated the higher the amplitude and the ESC will retard the timing until it diminishes. I highly doubt this is your problem though. It sounds to me like either A) you have a TPS problem or B) low fuel pressure.

Does the SES light come on?

[Lt.Del]

No SES light after startup. I wouldn't think it is a low fuel pressure situation...after the engine warms up, it even idles rough.  All of this happens after the engine is warm---I know the ECM sends signals for checks after the engine is a certain temp. 

What is the TPS system?

Where is the ESC module located?

If pic C is the coolant sensor, what is this, b/w the number 1 and number 3 plugs......?




thanks for the reply

[VileZambonie]

That's your coolant temp sending unit for the idiot light or gauge.

The ESC module is either right next to your EGR solenoid or mounted directly on the firewall but since I didn't see it on your EGR sol plate I assume it's on the firewall.

The TPS is the throttle position sensor but if it's doing it at idle and there's no SES I doubt it's that. Just for the heck of it jump the A&B terminals in your data link connector and see if it set any DTC's.

Just check your fuel pressure though... try a few things if you don't have a pressure gauge, find the flexible part of the fuel line and pinch off the return line (this should make your fuel pressure rise....) see if it gets better

[Lt.Del]

Well I got a lot done on the Blu Burb today.  I was so convinced it was a timing/ignition issue.... I could tell some cylinders were not firing.  I bought new plugs, coil, distributor cap, rotor, and today, I replaced the distributor ($105)....



And while certain I timed it right with my timing gun, she just would not run right at idle.  I had to plan my next move--obviously the problem doesn't lay with the distributor or timing or ignition in general. Next area of focus: fuel (thanks Vile).  On an earlier post, I mentioned it seemed she was running on two cylinders.  Well, actually, she was running on four cylinders.  The reason....while contemplating my next move after putting in the distributor, I looked down into the throttle body while it was idling.  I shined the flashlight down....I couldn't believe what I was seeing, or more specifically, what I was not seeing.  While i saw gas flowing in the right bore, I saw nothing in the left half of the throttle body.  There was no gas coming in that side! 



She was running on only four cylinders (if it is a true dual plane intake).  I played around with the wires and plug, pushed in on it, and VIOLA!! Gas started coming down now and the rpms moved up.  Perfect!!

I don't know if it was loose or what, but by pushing in on the plug, it started running gas again.  She purrrrrsss now.

Thanks for the replies.  I was so certain it was an ignition/timing problem.  Atleast now i have a new distributor, Cap, Rotor, Plugs.  She's tuned well now.    And, I have an extra good working distributor just in case I need it one day.  I am not gonna turn it in for a measly $25 core charge.

I'm ready for camping season now....life's good.

[VileZambonie]

Well at least you got it fixed. Just a side note - when your truck runs like crap be very careful looking down that throttle body if it backfired you'd get seriously hurt.  Happy camping!

[Lt.Del]

My wife would probably say I could use a face lift 

[VileZambonie]

lol