I have a 95 tbi 350 that I recently hooked up to my laptop with tunerpro to try and find out why I have low power and bucking while going up hill and found that my timing advance shows 16 at idle(confirmed with timing light) and as I accelerate down the road it goes down to zero as it starts to backfire through the throttle body, but if I get it past 3k rpm I can put it to the floor and get fairly decent acceleration.

Motor seems to be mechanically sound, but can't figure out why the computer would be pulling timing with a stock computer(16197427 with BYJL prom). Any help would be greatly appreciated, no suggestion too small

 

First I would set timing. Must disconnect timing wire, then set timing to 0. Turn off truck, clear codes and restart.

Timing should be a little higher than 16 at idle but does drop as soon as you accelerate so that part seems normal.

Popping through TBI has to be fixed to run right. When it pops it is raising pressure from vacuum and ECM is confused on what to do? Could be a vacuum leak casing this.

There's an EST module in distributor that does most timing part, it is also a known intermittent failure item. Still run, but not right. If you have an auto parts store that can check it ask them to do it three times. If it fails any of three times it is in it's intermittent failure stage. If you remove or replace use the thermal heat transfer paste under it.

HTH!

 

Knock retard? (check data stream).

RBob.

 

I have the timing set at 0 with EST wire disconnected, distributor is new as I had too much vertical movement of the old one(about 1/8-1/4 inch movement cold)that caused the rotor to rub material off the cap.

here is the engine idling graphs:


here is a blip of the throttle:


and here is a stall:


these were all done just in gear with brake to load up the engine. Really appreciate the replies.

 

I forgot to mention I'm not getting any knock counts during these logs to account for any of the timing being pulled out. Also there is a definite miss at idle that I haven't been able to locate the source of. All plugs look ok(nothing flooding). Plugs and wires were changed with distributor, cts is reporting accurate temperature(checked with infrared thermometer), and truck was warm and in closed loop when this was logged.

thank you for your time, and sorry for the long posts.

 

Do a compression test of cylinder that has a miss. You can't believe any data to be correct if you have a miss, exhaust gas unburnt throws O2 sensor into making wrong decisions on steroids...

How many miles on this motor? I've got 154k on mine and no play in distributor, seen motors with 250k and tight distributor, why did your distributor wear out?

I think your Manifold pressure in monitor is battery voltage and it's low at idle. That truck should idle close to 14 volts. 11.xx volts is to low. but then it went to 15.xx volts when stalled? So maybe that is manifold volts or kpa but the monitor is wrong? If the monitor settings are wrong and that is a MAP reading it should be steady, yours shakes.... back to compression test of misfire cylinder.

 

My truck has 140k on it, but I don't believe that my distributor really wore out as the pickup coil was not an original part. I think that the previous owner "fixed " a bad pickup coil and didn't put any spacers back in the distributor after changing it. I put in a new one because I didn't want to risk what else may not have been done right to it.

That makes an incredible amount of sense what you are saying about the 02 being way off because of the missfire. I have done a compression check and I got even numbers (160 lowest and 170 highest, average was 167).

The voltage on the MAP sensor graph isn't scaled right in the xdf that I was using....it is 1.1 volts at idle and 4.6 volts engine off. My actual idling vacuum is 18 with about a 1/2 psi drop with the miss.

I also just realized that I mislabeled the first graph....I was giving it steady slight throttle under load to show the big reactions to a little bit of throttle while under load....I can't find the idling screenshot that I did right now, but you can see just the start of it in that first screenshot.

While I'm at it I should mention that at the highest throttle position in the first graph equated to about 1700 rpm all while holding the truck stationary in gear with the brake.

Really appreciate the information so far. Thank you for your time.

 

You do realize that the stock calibration (BJYL) at low RPM and high load the SA does go to 0? May want to check the data log via RPM & MAP and match it to the SA table.

As for the stalling, I've seen this when the injector firing goes async and leans out. And the engine dies.

RBob.

 

I didn't realize that about the SA going to zero....the electronic controls education that I've had is pretty basic with more trouble code troubleshooting spreadsheet running than the actual nuts and bolts of ECM function.

One more quick educational style question, what would cause the SA graph to show vertical lines on the scan as I blip the throttle. I'm assuming it's a result of one of the variables that determine the SA value is out of limits, but not sure. And is there any resource that y'all know of that could bring up my level of understanding of what the ECM controls are doing to control the engine. I've gone through the tables and I'm not seeing the bigger picture of what it's trying to accomplish. Timing wise I'm used to the mechanical advance with high vacuum being helped by the vac advance on older motors...I'm not really getting any understanding of less timing at more rpm I guess. Don't want to waste too much of your time, but ever since I bought the truck and started scanning the datastream I've been really curious about what is actually going on in the little black box if you will

Again thank you for your replies and I appreciate you taking your time to answer my questions.

 

I wasn't able to understand it fully until I went through some hacs of the code. By reading it and seeing what the ECM did I was able to understand what the ECM was doing and what needed to be changed to tune the engine.

Then started to modify the code as the stock stuff didn't cut the mustard with a modified engine.

Not an easy to do answer, but that is what it took. However, this was quite a few years ago. I did some write ups on various areas of the ECM functionality. Originally they were notes to myself, but found it better to publish them.

Hit the tuning guide book sticky on the DIY_PROM board here. In chapter 5, under the '8063/'7747/'8746 ECMs there are several. One describes the SA.

Open the main open throttle SA table and it resembles a mix of centrifugal and vacuum advance. As the load increases (higher MAP) the SA decreases (vacuum advance). As the RPM increases so does the SA (centrifugal).

One item that was difficult at first was that MAP is close to being the opposite of vacuum. Many years of using vacuum, then all of the sudden MAP, and absolute pressure at that.

The reason for the SA spikes in the data log is because the SA is going after the base timing (relative to the reference pulse). The ECM uses a 2's complement number when this occurs. And the display program isn't converting it properly, which is actually difficult to do. As two different equations are required, one for before the base setting, and one for after the base setting.

RBob.