1994 LT1 with aftermarket harness, VATS, EGR and MAF sensors programmed out. Will start quickly when cold and goes to about 750 RPM but will not stay running and will not take throttle. After several starts it will hunt meaning it will almost die and then the RPM jumps to 1000 then down to 400 or so then back up but dies again. Once it warms up and goes to closed loop everything works fine. BLM is steady at 128. Can't put my finger on the root cause. Any ideas?

 

what's your fuel pressure doing? I am wondering if opti is acting up.

 

Fuel pressure stays at about 40 PSI. Fuel pressure regulator and Opti spark are new. Once it goes into closed loop everything is fine so I'm thinking it has something to do with the program. ECU was programmed by a third party which may or may not have lost info being passed on. I have two error codes. First one is knock sensor. My engine has one one knock sensor and the ECU thinks there is a second one. Second code is the MAF sensor. It's been programmed out and as a test I put a new one in and plugged it in. Still have the error code and no info being passed from the MAF sensor. I do have the Tuner Pro program on my laptop but having trouble understanding how to get it to work (age problem).

 

Did it ever run right? What type of Opti? The china ones do have a high failure rate even when new. The net is full of stories guys installing new units that were bad.

As you say, it could be a program issue. The open loop map could be off but keep in mind when you program the MAF out the MAP/BAP and the Air temp sensor. become more critical. While open loop does not rely on inputs from the O2s it must use some sensor inputs. Also open loop does not do well under extrema air temp variations. GM spends a lot of time an money getting the open loop map right for the stock motor. If a tuner creates a new Mapping at 80 deg F it may not work well at 10 degs. or 115 degs.

Way back in the 80's on the first MAF V8s there had a very high failure rate for the MAF sensor. It got to the point after a year GM would swap in a different PROM that programmed out the MAF. I can remember on one car it ran a little rough after the swap. Turned out the Air temp sensor was a little off. I guess with the MAF hooked up it did not rely on the air temp as much. A cold air charge needs a lot more fuel then a warm one or you lean it out.

 

I got the engine on line so I really don't know the history of it BUT the internals are as clean as new. I suspect there was a resent overhaul and MAYBE a cam change. Engine runs fine with no smoke or rattles/knocks. Just the open loop issues. Got to learn how to use TunerPro.

 

I assumed since everything ran fine when the system went into closed loop the engine ran fine. A small cam (stage 1) can give it a chop but should start OK.

I was wondering about the PCM system. There is a lot of non-factory stuff going on and I was wondering if it has every worked together, maybe on another engine? Did you ever have this system running a car well.

A vacuum leak would explain your issue but it would result in higher RPMs in close loop or the IAC zeroing out.

 

Adding to the cold start problem. Found out if I unplug the MAP sensor the engine starts right up and settles into a smooth idle at about 700-750 RPM. Also found out that the engine will take throttle nicely also. Figured I had a bad MAP and put a new one in but the original problem still exist. Will check the voltages on the plug tomorrow.

 

Would be nice to see what the PCM is seeing. Ever use the ELM 327 OBD2 readers? They are super cheap. They are no replacement for HPtune but they can log data right to your phone. Either way you know you are on the right track. You can drive it with the Map unplugged but in the end it may cost you a set of O2 sensors if you drive it too much. In open loop the system will not throw timing at the motor so as long as its not running super lean you will not damage anything. If you get a back fire or miss under load then just easy it home. More likely just a little carbon that will need blown out with a heavy right foot . In general open loop maps are rich so nothing gets hurt on the way to the dealer.

I wonder if it is tuned for an aftermarket 3 bar map sensor that can sense pressure for boost. Did you verify the part number of the one that came off?

 

Gorn thanks for the info. The MAP sensor is factory as well as the replacement (that produced the same results). My ECU is a 1994 Camaro # 16188051 which of course is OBDI. I can use EEHack to read and log data. It shows two error codes. One is the MAF which is supposed to be programmed out. To see if it would make a difference I added one to the inlet but it still just reads "0" which means to me that it (MAF) is not a factor in the program. Why it sets a code I don't know. Second is missing knock sensor. My 1994 engine only had one from the factory. I thing '95 and up had two which makes me wonder what year tune I have.

Three programs that I'm trying to use are EEHACK, FLASHHACK and TUNERPRO. Being old and not too keen on using the programs it's an uphill battle. Can't seem to wrap my head around the definition files and BIN files that needed to be loaded in order to see whats going on with my ECU program. Steep learning curve for me and the tutorials zoom thru it so fast I lose what I need to do and HOW to do it.

I have a show coming up this weekend across town. Think what I'll do is unplug the MAP until closed loop then plug it in and "go to town".

 

have you tried contacting Solomon https://www.lt1pcmtuning.com