What would cause the ignition timing to jump but not affect the running of the vehicle one bit?

I set the timing when I did my smog about 6 months ago. Did the whole jumper thing. I checked it again a couple weeks ago and instead of being at 10 degrees it was at 27 degrees advance. I never noticed any performance change at all so I have no idea when it changed. The only reason I checked it was because I was messing with my idle adjustment. I went ahead and retarded it to 10 degrees and the thing ran like crap, had no power. I put it back to 27 degrees and it runs as normal. I thought the timing marks slipped on the pulley due to a bad harmonic balancer so I replaced that last week and it wasn't the balancer. The new balancer shows the timing still at 27 degrees. I thought maybe my timing light was bad so I borrowed one from a friend and it still reads 27 degrees advanced.

I will add, I have had an intermittent stalling and surging idle which I thought was due to my brake booster being bad. The brake booster is bad but that wont effect timing. Just wanted to bring up the stalling/surging issue in case it could be something electronic.

I am at a loss here. I never changed a thing and its suddenly at 27 degrees!

Open to any ideas on what to check next. I have a factory service manual so should be able to trouble shoot most things.

Evan

 

Bad TPS, not showing 'throttle closed'???

 

Millball, I was kind of wondering about that. I haven't checked it recently since the problem started. So if it thinks the throttle is slightly open then it may be advancing the timing? And if its malfunctioning it could cause the surging and stalling at idle?

 

The conditions for the ECU to go to "base timing" are (1) the jumper installed in the diag port, and (2) the TPS switch showing closed throttle. If either of those conditions is not met, the ECU will manage timing according to its internal map, and a 17 deg advance at 1000 rpm would be reasonable.

I agree with millball, check out the TPS setting, and/or the connections from the TPS back to the ECU. It could be as simple as a sticky throttle cable/linkage that doesn't allow the throttle to completely close. Only the last couple of degrees of throttle closure will (or should) activate the TPS switch. You might try just forceably closing the throttle and see what happens.

 

Thanks for the info guys. I will look into the TPS more. I did force the throttle body fully closed last time I checked the timing. It made a slight difference in rpm but it didn't change the timing.

Could the IAC tie into any of this somehow? I am thinking I will pull the throttle body and clean that when I check the TPS.

 

TPS checks good. Unfortunately I don't know if was actually in adjustment because I was checking it wrong at first then loosened it up before realizing my error. I will pick up a new gasket and re-install it tomorrow.

If it still has the issue I will check the connections back to the ECU.

Anything else I should consider?

 

Well then, you still really don't know anything about what it was doing then, cause you must determine whether or not it was sending 'throttle closed' when at idle.

 

Yeah I was bummed I screwed that up. I forgot to apply vacuum to the little idle up valve. I don't remember what that thing is called. It was holding the throttle open slightly.

Well it's properly adjusted now so I'll know tomorrow afternoon.

 

Something to check/think about, vacuum leak. I was chasing some things around and then you will see the solution at the bottom. You may not have the exact same place leaking, but somewhere.
https://www.yotatech.com/forums/f116...stment-293290/

 

Good call. I will check the idle screw o-ring tonight. I have checked and re-checked vacuum lines and haven't found any leaks yet. I replaced a bunch of vacuum hoses when I rebuilt the motor so a lot of it is new.

 

Cleaned the throttle body, adjusted the TPS, Inspected the oring on the idle screw. New gasket, put it all back together and it worked! I checked the timing and it was right back at 10 degrees. I drove it a little tonight and it feels smoother off idle. Although it didn't stall it did start surging when I was sitting at a light for awhile. I will put about 70 miles on it tomorrow and then throw a timing light on it again and see if it is still showing 10 degrees.

Thanks for the help guys.

 

Run your heater on hot for a bit and make sure the coolant is topped up full.

Surging idle is sometimes caused by low coolant, or an air bubble in the system.

 

Coolant is full, I ran both heaters with my truck on an incline(front up). I am still having an issue where if I sit a long light the idle will slowly start to surge to where is surges about 500rpm. It never dies and if I blip the throttle it will stabilize but it will start surging again if I sit there.