I am trying to learn to PROM tuning on my quasi-stocker in preparation for an engine swap hopefully this summer. So, I've just started logging data from my 91 305 TBI. I spend a lot of time reading these boards, but I would really appreciate some specific advice about what my log data means. I'm not asking "what are BLM's, etc." But rather, what do my specific BLM's etc. tell you about my car?

I have read most of "How to tune and modify Chevrolet fuel injection," (by Watson) so I have a basic understanding of the terminology. I just don't have the expertise to interpret my data logs. Like, what would you guys do if your data log looked like mine? How would you change the PROM content to improve performance?

Anyway, here it is data log . If you could take a couple minutes to look at it and give me some feedback, it would really help me. Thanks, and I am always impressed by the technical depth of the posts on this board.

 

Take notice to your BLMs going up (showing slightly lean) right when you input throttle AND/OR the IAC opens up to a high number. You can correct this one of 2 ways, either way works. One way is to simply put a little more accel enrichment in for either delta MAP or delta TPS. aka "pump shot" tables. The other way is to increase the adder associated with the iac opening, this is sometimes important but for you, start with the AE stuff.
If you aren't burning your chips yet, at least give your fuel pressure another 1/4 turn or whatever brings your leanest BLMS to under 128, I've found this to be better for gas milage than BLMs in the low 130's, yes, I've got data to back it up believe it. Although highway mode was evoked so take it for whatever it's worth. Anyways, everything looks pretty good except for what I had stated above. Also, if you burn your own eproms, it would be interesting to get data about your SA. Replace your prom id msd or lsd with final SA. If you need help doing this let me know through this post and I'll dig up the thread that went into greater detail.

 

Quick question on the BLM's and INT.....less than 128 is RICH and more than 128 is LEAN???? and the ideal A/F ratio(stock) is set around 14.71 right? My car is having an awful time...cant figure out what to check from the data....lol Just thought Id make sure Im reading these articles right.......

 

you are correct sir

 

Thanks...not trying to hijack the post...just tryin to get some insight on my cars RICH problem.....my BLM's wont come out of 108 and cell 4 unless theres at least 80% throttle......

 

I'd be very interested in getting some data about my Spark Advance. That would be nice if you could shoot me a link to that thread you mentioned. I don't actually have the equipment to burn my own chips, yet, but I'm ready to buy.

One question: Since I modified my intake and did the open element, I have a sort of rougher idle during open loop. It's not like it wants to die, but it feels rough for a few minutes when starting cold (45F). The BLM's are locked at 128, so how can I analyze this open loop problem? I have a log. It's at base timing 6btdc.

Also, I hear a kind of whitle/sqeal often during partial throttle acceleration. It's not like a belt sqeal, and I wonder if it's the IAC. Do the IAC numbers look normal? I read in Watson's book that there should be 256 IAC settings (from 0 to 255). I only go to 128. Is this right?

JPrevost, thanks for taking time to help me. I want a nice TBI 350 similar to yours.

 

those are 256 steps in IAC. I dont think you will ever open up to step 256. i believe GM has available if needed for another application. there is a max IAC value set in your eprom that most likely is different than mine and it is not 256. now i was told to begin data logging at idle and i assume closed loop if working on ve tables. then i would find a level road and maintain a constant speed and try to get readings in all all areas of map without too much movement of pedal. i quit at 3600 rpms as was told not to be too concerned with above that. i may disable AE to 85% tps and try to look at 4000.

 

Mr Prevost, Jon, can you explain why his IAC's are moving around that much? seems to open close based on counts. i assume he is in closed loop. why the need for ECU to open/close that often? does not IAC move to its minimum position when in closed loop and only move when air cond is turned on or we go drive/neutral/park or ??? I thought once engine hits operating temps the ECU does the A/F BLM correction with INT ? am i misunderstanding since i have SD 7747.

 

After reviewing that second log I posted, I see that right when I start the car the IAC is 191 and after 45 seconds ramps down to ~130. After about 5 minutes it seems to hit a steady state at around 20 +/- 5. Hmmm. I wonder if the rough idle is related to the IAC...

Also, in closed loop, I get IAC sensor value of 128 (apparently pegged) with as little as ~20 percent throttle (Time~900 s). This is the same (IAC=128) as at WOT (TPS=98.x). Is this normal? Like Ronny said, it seems kinda jumpy, but then again I have nothing to compare it with. Maybe someone experienced could straighten me out...

Ronny, thanks for the pointers on collecting log data. I'll get one like you explained.

The car seems to run well after it warms up and goes into closed loop. I'd really like to resolve the cold idle issue. I'll look at my repair manual and see if I can figure out how to test/adjust the IAC position.

 

in closed loop settling in at 20 i would guess is normal. i bet the issue is elsewhere. check engine light come on? maybe needs a good tune up. would not hurt to check fuel pressure. how old is fuel pump?

 

Ronny, I agree that IAC settling to 20 at idle sounds reasonable, but 128 at 20% throttle might be high. I don't know, though.

Anyway, I pulled a max ~160 rwhp a couple weeks ago. If you believe the 20% loss through auto-drivetrain, then that's 200 at the crank. That's 30 hp over the stock rating for my car. So, I don't think that I need a good tune up. I could be wrong.

Fuel pump is stock, so 58000 miles and 13 years on it. No error code. I've never got a check engine light other than code 42 when setting base timing.

JPrevost, I think that I found that post that you mentioned. Is this it?
If so, it's a little over my head... Could you maybe give me a little more background? Like, what are lsb and msb? One thing I thought I gathered from that post was that SA lives in the ECM not the PROM. So does that mean that I don't need to burn a chip to get at the final SA? Can I get WINALDL to output it?

 

Your IAC step counts sound normal. High durinng crank/start, then tapering off as the engine warms up. The steps do increase with application of the throttle. This is a function termed thottle follower (TF). Again normal.

Many folks like to change the gain and max TF steps. Can check out some of the recent threads for discussion.

RBob.