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 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!!

Thanks for any help in advance
Tedd Edmondson




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!!!!

 

Your IAT should be somewhere on your air intake. Generally its close to the front of the car. If the IAT is reading -40 then I would suspect its unhooked or there is an open in the circuit. Seems to me all the GM cars and trucks I've worked on -40 was kind of a default temp, meaning if you unhooked the sensor the scanner would read -40. I believe this to be true on the ECT and TFT sensors as well. Being as its a 95 model and OBD1 (I think) it may or may not set a code.

 

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

 

The IAT will not go in the intake plenum. It should go somewhere in the air intake tube or airbox. Have you removed the airbox and looked on the bottom? The IAT measures the temp of the air entering the motor. On ODB2 cars a skewed or unhooked ECT,IAT, or TFT can set Ratio codes and or voltage codes. If the ecm sees a ratio problem the car will never enter closed loop and you will lose performance and gas mileage due to little spark advance and rich fuel trim. If you cannot find the hole in the airbox or intake tube I suggest just installing a new IAT in the intake tube and hook up the connector. It just needs to have air the motor is sucking in passing over it. Good luck!

 

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) is running in closed loop and has a reading of -40° which is equivalent to over 100,000 ohms resistance or an open loop condition for that sensor.

Thanks for you help so far
:hail:

Climbing the learning curve to enlightenment,

Tedd Edmondson

 

I looked online and could only find 1996+ trucks with an IAT sensor, but was able to find a picture locating it on one. It is supposed to plug into the drivers side of the throttle body apparently. This picture is showing them relocated, so the wiring should be on the other side of the picture, opposite where the arrow points. Is that where your wiring was found?

 

The IAT doesn't measure the amount of air thats coming in, only the temp. Did you change the ecm or wiring harness? The ecm takes the the data from the IAT, ECT, MAP and other sensor to decide how much fuel and ignition timing is needed. If your ECT is showing 80* and the IAT is showing 80* then the ECM knows to feed a lot of fuel and idle the engine up for warm up. If your ECT shows 80* and your IAT shows -40* then the ECM gets "confused" and defaults. The IAT is nothing but a glorified resitor. The resistance changes as the temp changes. If you have the connector buy an IAT and plug it in. Don't worry about installing it into the air intake yet, just zip tie it up so it doesn't get in the fan. It should fix the problem. The IAT just has to be able to measure outside air. If it fixes your problem then take the time to install it. Good luck!

 

If this doesn't help let me know. Tomorrow I can look up your truck on ALLDATA and find out exactly .

 

Seems to me like all the TBI trucks had a IAT sensor in the air cleaner base...

You're right, -40 degrees is the lowest temp it can read, and is what it will say if the wire is unhooked. If you look through some of the other boards you will see that some people use a resistor to simulate some sort of reasonable but low temp (50 degrees or some such) which might also be a good approach in your case.

 

Yes, I have found some posts that give resistance values(ohms) for different ambiant temperatures for the IAT. 1600 ohms is the resistance value for 100° F intake air temperature.

Another clue I have just found is on the Datamaster software package, with the correct definition file installed ($0D) there is a sample file labeled [C:\...ls_95GMC_eng_trans.uni] and under the comments header, reads"test run in queen creek", well guess what the IA Temp reading is, .... -40°

Of course this leads me to believe that the Datamaster test vehicle had its IAT disconnected or, more likely, 1995 GM trucks don't come with an IAT sensor!!

But that blows my theory that the missing IAT is to blame for the less than stellar performance I'm experiencing.

It may be time to start adjusting my timing and fuel tables on mt PCM.

I already have a Craig Moates EPROM adapter, but how do I remove the original PCM without destroying it and its calibration file. There are 28 pins (14 on each side). Do I heat each pin and suck the solder out with desoldering braid and expect the PCM to just slip right out after the process was repeated 28 times? That seems a bit optimistic to me.

Thanks for all your help,

Tedd Edmondson