Hey, so i was driving my truck and on the highway, it was jerking and bucking really bad at certain throttle and rpms, it was even backfireing out the exhaust sometimes!!

So i had to take a 45 mile drive, and i hooked the laptop up to see what was going on once and for all

(keep in mind, when tuning this thing, i may put like 10 miles on it in 1 month then drive it alot, and let it set, so its hard to remember what i did last tune and whatnot....)

But anyways, i spent forever driving down the road and looking at the laptop trying to see what was up and I finally found the pattern, I had made some decent timing come in when im at highway speed and map, so everytimg the timing reached, oh, 36.8 or so and up, is when it would do it.

My question is why???? I have the narrow tbi distributor, is it actually possible the timing was jumping to the next cylinder? thus why it would even backfire at times? (again, if id consisantly drive this thing, id realize, ok i turned the timing up and now its messing up, that has to be it, so sorry if i sound stupid not realizing thats what it was at first)

So to get that much timing, I take it i have to get actual physical distributor advance, then run the rest in the tune? Because i sure didnt think 37-38 ish was that extreme.

But im sure glad i found my missfire issue.
Thanks.

 

What is your base timing? The ECM calibrates off of that, so... if the base timing is wrong the ECM will calibrate off the wrong values with the wrong assumptions.

 

My base timing is 0, and I started with a stock tune, which should also be calibrated to 0....unless somthing funky went on and my distributor moved, but i doubt it.

 

Does the 0 degrees take your cam into consideration for base timing? I've seen some SBs where 32 degrees is the max for advance at rpm's over 3k. It just depends on your combination, compression, etc. 36 sounds a bit much.

I went through a devil of a time figuring out what my base timing should be given my engine, mods, and MEMCAL calibration. If your cars seems to drive great in closed loop, then runs like garbage and trips a code 43 (knock sensor), so may have it defaulting to the limp home mode. My ECM was failing its self test because it could not induce knock because of retarded base timing. Took some experimentation to figure out the best setting but I got it.

What else have you found out from datalogging? Do you get a fast or slow MIL when it runs rough (closed loop vs open loop)?

 

believe me its not in limp mode. my engine is basially a stock spec tbi 350 with an LT1 profile cam.

Btw, its not a, start the engine, and it will run rought all the time sorta deal. ive got it narrowed down to the peak timing, becase it will ONLY do it at the same RPM, vs MAP no matter how fast im driving, and when i give it more or less gass the miss dissapears, and like i said, the time it does it, is when my timing was set about 4-5 degrees higher than all other tables because it was right in the highway zone and i was trying to achieve best milage for the power tour.

Im gonna do the math and redo the fuel tables to get them closer to 128 and take the max timing on my timing table to about 35 degrees and see how she does tonight.

 

What about your fuel pressure? How about your cap and rotor?

I have seen some coils peter out at higher RPMs.


FWIW... LT1s run about 38 (total) degrees for the timing at 4,100k. What heads are you running?

 

lol. stock tbi heads dude,

anyways, its fixed now, im pretty sure....Had a 30 minuite data log yesterday and i redid the fuel tables according to that, then i set my highest timing number to around 35, and it ran good.


First thing i did was got cap rotor and module a while back because it honestly felt like an ignition miss, and i couldnt hook my datalogger up back then.

Now the only main issue i have is in open loop timing, i have no idea how to do that in tunerpo, it runs so rich it will backfire, but i have it tuned to stay in open loop for 10 seconds...

 

this sounds very similar to what my tune was doing. I took some timing away and its been fine since. Its interesting how some guys get away with so much timing around here. My engine is just the opposite. At part throttle and light load, my engine only wants about 34-35deg of ignition advance. in 80* weather with Sunoco 94, my engine only wants about 28* at WOT.

 

Ya thats what threw me off, i coulda swore people were running more even on stock motors, maybe not. But anyways, im still glad its better, i got an ever so slightl buck when it hit 35 degrees of timing a few times so i may even back it down a tiny bit more if it still does it after i redo the main fuel table yet again.

fun fun

 

Spark timing is dependent upon the heads, compression ratio, cam timing, vehicle weight, convertor stall, and a bunch of other stuff. Check some stock truck calibrations. Having 11 degrees of WOT timing at 2200 RPM isn't unusual. Cruise timing at that same RPM being in the 18 - 22 degree range.

RBob.

 

I`m haveing similar problem.I`ve got 32* by 2800 and 34* at 3200 and above.I`m getting up to 12.5 degrees of knock retard.I thought with alluminum heads and 93 octane I could get even more but I`m picking up knock counts and getting all of this retard.I dont hear the knock myself ,but from reading my plugs I`ve determined it to be true knock.Dropped my plugs down 2 heat ranges and it`s still there.All I can think of is it`s just too much timing.With the zz4 heads my c/r should be about 10.5:1 which should also be ok for 34*.Why do some engines not like the timing and others dont?

 

With the ZZ4 heads ('113 casting, not sure about the newer ones). Drop the WOT timing to 24 degrees BTDC. Then work up from there. Probably find it needs to be in the 26 - 28 degrres BTDC range.

There is another reason that it may look like the engine can use more timing. This has to do with the ignition module. Some modules retard the timing by up to 8 degrees. Which puts a true 26 crankshaft degrees at 34 degrees in the ECM tables. Do a search on "latency AND module" with Liquidh8 as the posters name.

Also, newer heads have a faster burning chamber. This reduces the spark lead requirement. The L03 & L05 heads have swirl ramps, again, very little timing is required.

RBob.

 

Even if I have one of those modules that doesnt cause the knock.I`ll look at the module to make sure iteasnt replaced with one of those 043 ones or whatever.These knock counts and knock retard are killing me.I`d like to run some nitrous but not till I get these knock counts figured out.

 

hey guys, it seems as all this just started after i replaced my ignition module, maybe, maybe not, but anyways, it had what ever mod that came with a summit distributor, and i replaced it with a module for a 92 1500 silvy... Anyone know if that is an ok one to use

 

If you are worried about the module latency, then I suggest you do what I did. Which is to measure it. It is all described in the thread I mentioned.

As for the actual, true, at-crank timing, give the engine what it wants, Grumpy (tm). Both the '113 heads and the stock Blazer heads will not want the amount of spark advance that is being thrown at them.

In both cases the engine is also telling you this.

RBob.

 

I will agree here 100%. The 305 in my van with either 081 TPI, 187 TBI, or 059 Vortecs likes NO more than 26* @ 4,000 rpm with a very mild advance rate. It flat gets it too.

 

Thanks guys I`ll get it backed off some and see if I loose my knock.I hope that does it.Thats really all I had left to do with my limited knowledge.

 

Im having a similar problem with my stock 87 l98 at normal throttle between 2 and 3k rpm it jerks until it shifts into the next gear, i have no idea what it is

 

Reviving or amending this thread to ask if it`s normal to have the advance lower than stock to eliminate te knock.I got it back to 26* in the prom and I am still getting knock retard.It`s not near as bad as it was,5* retard now compared to 12.5*retard before,I havent had a chance to do much dattalogging just enough mopnitoring with a scanner to see if it was still doing it.I know yall said to drop it back to 24* degrees and work up from there,but we put it to 26* degrees wondering about it being normal to go lower than stock.It didnt seem to run like a dog even with the lower advance,I`m assuming because of the lack of retard.Anyhow,Input would be greatly apprecieated.

 

are you using a stock or modified engine?

 

New l98 block hyper pistons,zz4 heads,LT4 production cam w/1.6 rr,Basicly stock tpi,headers full 3" exhaust,2500 stall 3.42 gear

 

I've been fighting timing lately as well (408 w/AFR 195s and a cam like a LT4 hot cam). I had some settings that I thought were running well but I was getting knock in lowers rpms and I was dropping lots of timing trying to get rid of it (car was getting lazy). So I stopped and decided to start all over again with a new bin and a spark map from a stock AUJP. Which actually had more timing in it then what I was currently using. I was expecting it to knock but it actually ran decent and I had drastically reduced (almost eliminated) the amount of knock I was seeing. I did have to make an adjustment at 1200 rpm and 40-45 kpa. I was wondering if having too low of timing was causing other problems. Also my AE tip in was decent although I experimented with adding more pump shot. That just made the engine stumble a little so I set the AE back. I think my problem was I was making too many adjustments at once and got lost somewhere. I am almost tempted to try a stock ANHT spark map with 3-4* dropped off to see if that works better then AUJP.

 

I`m thinking of getting a stock auto vette tune and see how it runs just for a referance.