Timing-IAT-Datamaster disaster!!

1995 4wd auto E/OD GT4 Suburban

I have replaced the original 5.7 motor with a 355 c.i. split rear oil seal short block, a Crane cam [part # 113931] with intake lift at valve .440" and advertised duration of 266° and exhaust lift at valve .454" and advertised duration of 272°, a pair of 23° twisted wedge Trick Flow heads, K&N air filter. 1 5/8" Flow Tech shorty headers with O2 sensor coupled to 2 1/2" dual exhaust with dual cats, mufflers and no crossover tube. The motor is topped off with some 8mm ignition wires. The rest of the motor and car is bone stock.

The motor has more power than the stock unit but something is'nt right. I'm ready to burn a new PROM but I want to make sure the ECM is functioning correctley first. I don't seem to be getting enough spark anvance, at around 3200 rpm with 75% TPS reading I'm having 13° - 17° total spark advance. I've taken my laptop with Daatamaster software to the local Chevy Dealer and snuck into the service area and spoke to some techs, they agreed that total timing should be higher and they said my IAT reading was too low (-40°F)and the sensor and or wiring was incorrect (i.e. open loop). This is a TBI and darned if I can find anyting that looks like a Intake Air Temperature sendor {Where the heck is that thing?} I get no codes, how could a sensor be bad or unconnected without a SES code?

By the way stock specs set timing at 0° btdc with the DST wire disconnected, which results in a motor that barely runs . After playing around with the timing I've finally settled on 8° btdc and that helps but still no advance beyond the 14° to 17° range at high way speeds under load!!

P.S. How do I remove my factory PROM from the ECM without destroying it and its data? Looks like quite a challange to me!!!!




Thanks for any help in advance
Tedd Edmondson

 

I found the wiring with connector for the IAT sensor, but no sensor to be found? There is a threaded hole in the #8 intake runner that looks like a likley spot for it to go. the motor in the truck is out of an 87 so that hole has a vacum fitting for vacum hose but is not used and plugged. I was sure that when I went to the original motor I would find an AIT sensor in the #8 intake runner hole. When I looked, no sensor, only a square head plug? There is nothing on the air box(mounted on passenger inner fender), the tube that connects the air box to the air filter or the air filter. Do any of ya'll have access to a 1995 1/2 ton Chevy/GMC truck? Where is your IAT(intake air temperature sensor). I read a post that had resistor values for corrosponding temperatures, so tomorow I'll try to get some resistors to jump the connector with and see if I can change the -40°(default setting) reading that I am getting with no sensor connected. I know this isn't a fire bird or a camero but its a TBI 350 and all help is appreciated.

Tedd

 

Hmmm, I looked in my Chiltons manual and it doesn't show an IAT for the 94/95 trucks. Seems none of the online car parts sites list an IAT either.

However I also checked DIACOM and it lists IAT as one of the reported sensors. Confusing to say the least. The local GM parts counter should be able to clear it up. Its possible that connector you found is unused.

The reported timing your scan tool shows will not reflect any timing change you make at the distributor. The ECM has no idea where the dissy is set at.

Removing the PROM from the Memcal requires soldering skill and a fine point soldering tip. You might want to look into Craig Moates adaptor (prom board) if you don't solder.

 

I think I read on here that the Trucks didn't get an IAT sensor. I don't think your computer uses it. If it isn't showing an engine light, or showing a code for it, then your computer doesn't use it.

 

Yes it is a bit confusing. Information on IAT sensors is sporatic at best. I'm about convinced that my truck didn't come with one, but in that case how would you determine the amount of air comiing into the engine? MAP would only be half of the equation, density calculated as a function of temperature of the air would be the other half (it seems to me anyway). It's also confusing as to why all the MAP sensor F-bodys have an IAT sensor and that another GM product (albiet a much heavier one) with the same engine control system would leave it off! The Haynes service manual that I looked at for the vehicle mentions the IAT and shows a picture of a sensor in the pre filter intake plumbing, but that is not the configuration on my vehicle.

I'm not getting any SES codes with the connector unconnected, I need to find a stock 95 Suburban and look at its configuration. As I said earlier the Datamaster software loaded with the correct definition file for my vehicle ($0D) has a reading of -40° which is equivalent to over 100,000 ohms resistance or an open loop condition.