OK, I need some help. I just put together a '96 LT1 with ported heads and a GMPP #845 cam for a retrofit. I converted to OBDI via a #8051 PCM and a mailorder tune (VATS, CAGS, EGR, EVAP deletes, fans, etc.) I rebuilt the Opti with a new rotor, cap, etc.--super clean and fresh inside. I started the motor to run on a test stand (no alternator, coolant, etc.). A couple of issues are occuring. First, the motor cranks great until the ignition is triggered (i.e., I used seperate starter and ignition circuit switches for purposes of the test run). Once I switch on the ignition while cranking, the crank speed goes WAY down, similar to what happens when a motor is running too much ignition timing. Very hard to start. Secondly, once started, the motor runs like crap. Rough/poor idle, repeated stalling. Rough running/surging continues at partial throttle as well.... So, I need some pointers about diagnosing the problem. I am new to LT1's (and fuel injection for that matter). Will Freescan provide the diagnostics I need to determine if I have a spark or fuel problem? Any advice appreciated.

 

FreeSCAN will help greatly, you are not getting any SES lights or are you? It will show DTCs also.

If you run a log of various RPM stuff you can email it to me (zip it), I ran a similar cam and they should be quite tuneable.

 

Thanks Dr., I will probably take you up on that offer as soon as I can communicate with the PCM. Actually, I don't have the SES circuit connected or even an ALDL connector at this point (just ordered a cable and connector, should be ready for Freescan by the end of the week). The motor is going into a Jaguar XJ6C and is only set up on a test stand with open exhaust manifolds. Anyway, I did some basic circuit testing on most of the sensors, all check out OK, including proper resistance at each of the injectors. All the cylinders appear to be firing as evidenced by my "old school" timing light/tape on the harmonic balancer. The timing at idle does fluctuate an estimated 5 degrees or so as the motor struggles to run (not erratic, but a fluctuation, so I am not quite ready to blame the Opti). Fuel pressure is a tiny bit low at 40 psi (ignition only, not running) and expectedly drops to about 32 psi at idle, so, it seems like the pressure is not really an issue. I pulled all of the plugs, they appear WAY WAY rich and consistent.....carbon/black--probably the most telling sign in all of this. So my latest question--I assume the PCM is running open loop exclusively during my test runs? (the longest I have managed to get the motor to run is about 10 seconds) I assume all open loop parameters are tunable with Tunercat or LT1 Edit?? I perused Freescan briefly and it appears to have logging capability? (I kinda need this in light of the short window of actual running time...) And I still seem to think my slow cranking speed is coming from too much timing advance...

-Mark

 

A jag eh? You must be using a JTR conversion then

Open loop is tuneable with Tunercat or LT1 Edit yes, although MPG isn't going to be as nice, closed loop is definately preferable. I did run open loop however, I was in a time crunch and my home-made O2 extention was causing trouble with reads on one of the O2s (headers required an extention).

The injectors are rated at 43 PSI, most pumps are putting out about 43-45ish from what I have heard from others doing pressure testing, when running I'm not sure what the normal pressure is though, hmm.

The car dies after 10 seconds? Somewhat strange, but if your on a stock tune that is not too too supprising, actually I will bet its the IAC counts, and yes you can see this AND log it with Freescan, I used Freescan and Excel to get all my data collection for tuning the car. The stock TB might need to be worked on to help get the car idling, as well as some timing work with a cam that big! I ran an aftermarket TB which really sucks as far as how it is setup as it does not have a sealed IAC passage, really terrible setup.

Do not go overboard opening up the passage, even a little bit makes a good difference. You want to see approximately 40-60 for IAC counts, if you go too much past that (as you would with a big cam not getting enough air at idle) then the car will want to die because the IAC wont be able to compensate, maximum IAC counts are 160 and my car right off the bat was doing 160 and could not idle for very long without me giving it throttle (taking it out of idle and opening up the TB blades). I tried at first cracking open the TB blades a bit but this causes a vacuum leak at idle and is not the way to do it, plus may close it partially at WOT, a bandaid ONLY until you get the IAC opened up with the drill bit of your choice. I want to say that I used an 11/32" but I am NOT sure at all, measure the stock IAC hole and make a guestimate as to how far to go from there, remember though to go a little at a time, no 1/2" stuff or anything

Oh, it takes a couple minutes for everything to warm up enough for closed loop, so you are in open loop yes. Freescan will tell you when closed loop happens, you will see your BLMs move around during this time also (block learns/fuel adjustments).

Hope this helps, I will clarify if the above rambling is nonsensible...

 

Dr.--it's not a JTR conversion, not sure who originally did it. I bought the car with a 350/700R4, soon replaced that with a 406/T5, now going to LT1/T56 (death to all carbs in street cars). A luxo-rod is the concept.

Your ramblings do make sense, however, I guess I am surprised at the running results even with a stock tune? The cam is reasonably mild, 214/222 on a 112 lsa, although lift is .520/.544 (1.6 RR's) which may attack idle quality somewhat. My problems extend beyond idle, throttle produces surging and misfires, and a definite guaranteed stall when closing the throttle blades to return to idle. All something I would not have expected with a stock tune.

My cheapo Haynes manual states 41-47psi fuel pressure at ignition with a drop of 3-10psi running. My rich plugs seem counterintuitive to low fuel pressure, but maybe that's not a proper conclusion on my part.

Thanks millions for the links, I will definitely check all. It sucks that the motor's running a bit sour, but the thought of tuning with a PC has got me chompin' at the bit. Nice to learn new things after all these years.

What cam did you run when you did your TB mods? Which aftermarket TB? (I have an Accel 58mm sitting on the shelf for a later install.....)

I now recognize your name from the Advanced Induction site. I am humbled by those flow results. My home port produced a mediocre peak of 252 cfm at .550" with 2.00/1.56's, and only 5-8% gains at mid lifts (comparing to published stock flow numbers).....maybe I should be happy with these numbers considering the intakes are only 175cc. What were your cam specs for that dyno run shown on the site??

-Mark

 

Oh brother, I am so used to seeing the GMPP 847 that somehow I never saw 845 as you posted, oops.

214/222 should not be that rough with a stock tune, doh.

Not bad for a home port job, some people dont get much more than stock at all, yet others claim 260-265ish which would be great for a rookie porter, especially since alot of people pay $2,000 for that kind of end result

The cam is like an improved cc306, but its a proprietary setup, which I'm not a fan of (secrets) but it was in the neighborhood that I wanted, kind of the last wrung on the ladder of street car with stock rear gears, and hydraulic lifters, at least IMO. Some would go for more, some less.

From what I know, that is where the experienced hand comes into play, not so much peak numbers but the low-mid lift stuff, I used the same size valves you did, and was told besides new seats, that larger was mostly a waste on my setup, maybe a bored out car but probably more shroud at low-mid lift than anything advantageous.

Sounds like your issue is not going to be as easy as blaming it on the cam then, its not too far from what some people would run and never touch the computer at all. Run a log ASAP if that is going to be possible, thats going to give more clues than anything else I think.

Oh, I used a BBK 58mm TB (cheap), I heard that earlier almost all MFG followed the casting design of the stock unit as far as the IAC passage goes, yet later many companies changed because supposedly it worked better? This is far from the truth! Bad bandaid fix, ick, most MFGs use one of probably only a couple companies for the raw casting, complete the unit, and put thier name on it.

I tried a mail order tune, and it was worse unfortunately than what I used off the net as a baseline. This was my one and only experience with mail order tuning, and my particular roulette play was not in my favor, the timing was WAY off for what was effective in my setup and being off by only 2-4º can make a huge difference, most noticeably at idle and off idle power.

 

Dr.-- just sent you an email pertaining to this thread.

-Mark