Looking for ranges on Fuel Trims and Ignition timing

[2001ZR2]

Have Wifi OBDII BAFX 327ELM scanner I use with my iPad. Using the DashCommand software

Trying to figure out what is going on in my 1997 Suburban with a 5.7L Vortec, 3.73 gears, 4L60E and LT265/75/R16 rig.

My mileage is horrible in the range of 10-12 mpg so I am looking at the fuel trims and timing to find a clue on where to look next.

The STFT (short term Fuel trim) will vary from -7 to +7 but at a stable speed settles in around -3 to +3 the LTFT (long term fuel trim) will peg at 9.4 and will vary downward as the engine load varies.

Things I wonder about is the Original Ignition coil but the secondary resistance is about 6.5 kOhms. I will check the primary resistance again as I had it on the wrong scale. The ignition control module is also original. The truck has 309k miles.

I also wonder about the fuel spider which is why I checking the fuel trims.

Things I have done without really changing the mileage...New Magnaflow cats and y pipe, Mangaflow muffler and cat back, new spark plugs ACDelco Iridiums (old ones had gray from fuel additive but not bad) and new ACDelco. Replaced the PCV and tubing. Yesterday replaced the air inlet hat at the throttle body as the clip had disappeared.

Also cleaned the throttle body and EGR Valve with a new gasket.

Checked the vacuum and have just under 20 inches at idle.

Cap and Rotor are not that old at less than 2 years. Fuel pump is also less the 20k mileage usage.

Trying not to use the parts cannon but are those fuel trims reasonable?
and I see +33 timing on the highway.

Almost forgot the coolant temp goes up to 194 F and stays there.

 

[Schurkey]

I see +33 timing on the highway.

I don't know what reasonable ignition timing is for these trucks.

33 degrees of advance seems low to me, but that might depend on the base timing.

I'm guessing you'd want to see 32--36 degrees of "total" advance; plus 5--10 degrees of light-load "vacuum" advance. Of course, all the advance is electronically computed, there's no centrifugal or vacuum advance mechanisms--but their equivalent would be programmed into the ECM.

To me, "33 degrees" of advance is about 10 degrees shy when lightly loaded at highway cruise.

 

[AK49BWL]

Mine will not run more than 37 degrees at 1800-2200 rpm, the spark jumps to the wrong cylinders and I end up with misfires like crazy if I try to run higher. You figure the tabs are only 45 degrees apart in the first place.... 33 is quite acceptable on the Vortecs. Of course if the Vortec distributor systems weren't such JUNK..

OP: Have you cleaned the MAF sensor? That sensor is in control of most of the fueling activity in these engines, and if dirty, will tend to skew the fuel trims quite a bit, plus muck with your mileage. Also make sure your Intake Air Temp sensor is reading within +/-5* of ambient while driving, because that will mess with fuel as well.

If you're still running the original injector spider after 300k miles, it'd probably be a great idea to replace it with an MPFI unit (individual injector heads) even if it's not really bad.

Another thing is, the stock tunes on these trucks leave much to be desired.. A proper tune will net you a few MPG. I'm running my 97 with a 0411 computer tuned with HPTuners and pulling about 15 city/18-19 highway, and that's on a mostly stock 5.7.

 

[2001ZR2]

I did clean the MAF and IAT did seem close to ambient. Also replaced the filter filter as it had surface rust.

Also forgot with the new cats I install new O2 sensors front and back.

As for the spider I am planning to do that when I do the LIM later this year. It's starting to weep at the passenger rear corner. I'm thinking about getting enough fuel injector cleaner to treat the entire 42 gallon tank and see what happens. I am fond of BG44k which cleared up a dead miss on my son's 2001 ZR2 blazer

schureky and AK498BWL thanks for the replies

 

[Schurkey]

Mine will not run more than 37 degrees at 1800-2200 rpm, the spark jumps to the wrong cylinders and I end up with misfires like crazy if I try to run higher. You figure the tabs are only 45 degrees apart in the first place.... 33 is quite acceptable on the Vortecs. Of course if the Vortec distributor systems weren't such JUNK..

WHAT IS YOUR CAM RETARD showing in the data stream? (Cam sensor to crank sensor synchronization) Spec is + or - 2 degrees at ~1100 rpm. Wild Guess: It's WAY off if you're misfiring at 37 degrees. That, or the cap is partially-failed, or the ignition coil is weak. That was one of the nice things about the older, "large diameter" HEI distributor system--the plug wire terminals were spaced farther apart.

It is true that the distributor cap terminals are "45 degrees" apart on a V-8.

It is also true that the distributor turns 1/2 crankshaft speed. If you don't account for the speed difference, it's easy to screw-up the "degrees".

Ignition timing is (generally) read from the crankshaft damper, sometimes from the flywheel, but nearly always from something turning at crankshaft speed. "37 degrees" of ignition timing is 1/2 that at the distributor. Having the terminals 45 degrees apart is NO PROBLEM even with 50 degrees of spark advance, because 50 degrees of advance is only 25 at the distributor.

Put another way, the distributor cap terminals are 90 crankshaft degrees apart; as you'd expect for an engine design that fires a cylinder every 90 degrees of crank rotation. 720 / 8 = 90

 

[AK49BWL]

Cam retard is currently -2.8* at last check. I'm gutting the whole system in favor of CnP anyway. Tired of replacing caps after only a year of use.

Ignition timing is (generally) read from the crankshaft damper, sometimes from the flywheel, but nearly always from something turning at crankshaft speed. "37 degrees" of ignition timing is 1/2 that at the distributor. Having the terminals 45 degrees apart is NO PROBLEM even with 50 degrees of spark advance, because 50 degrees of advance is only 25 at the distributor.

Hmm, did not realize that, so thanks for the enlightenment but I do think the terminals being so close together is the real issue with the Vortec distribs.