Looking for some opinions from some advanced tuners like Rbob or Grumpy.

My problem is in cool weather especially but happens to a lesser degree all the time. With a warm engine shut it off and let it hot soak for a period of time. Upon restarting the engine seems to run lean for a few minutes and then will be fine. The scenarios could be stopping for gas, stopping for an ice cream, etc. When you come to leave the car has no power. My car is a stick so it's either over rev the motor and slip the clutch or kill the engine a couple of times. Either way you look like you've never driven a stick before. Believe me, I driven sticks for the better part of 40 years and this is the damnedest thing.

I'm running a '7730 with an 8D mask. My diagnosis is this. While the motor is hot soaking the MAT sensor goes up as there is no air moving past it to cool it. Then when you start the motor the cool outside air (my car has cold air ducts) rushes into the motor but the MAT is still reading high. The ECM reads the high MAT and adjusts accordingly but the air temp especially at this time of year is 120-130 degrees cooler. This results in a lean condition and no power. A couple of engine stalls and then the revving and slipping clutch routine to impress your friends as you leave the drive in and you're on your way. After a couple of minutes the MAT and the air temp stabilize and the motor runs great again. By this time you are 2-3 blocks down the street and you can let the clutch out without even touching the gas. But there is no one there to witness this feat of stickshift driving/engine tuning wizardry. You've already made your impression leaving the drive in.

Does this seem like a likely diagnosis of the situation? I've noticed in the summer that the BLM's are rather high after a hot soak restart and gradually come back to a more normal level after a short time. And if this is in fact what is happening does anyone have a cure? Thanks.

Regards,
Gary Anderson

 

The MAT does have a very long transient response, so this could very well be the cause if the MAT is mounted on or near the intake plenum.

The MAT should be mounted as close to the inlet of the CAI as possible. This will report the incoming air temps. Youll then have to rework the MAT correction and MAT/CTS vs gms/sec airflow blending multipliers for the SD calcs so the correct fueling corrections are applied. The air density is estimated from both the IAT and CTS. At low flows, the CTS mostly reflects the incoming air temps as the air has time to warm in the plenum. At high airflows, the IAT will mostly reflect the air temp as the air is rushing in too quickly to have time to warm.

Also make sure your using a cage type. This has the sensing element freestanding in a cage, which insulates it from the heat of the engine compartment. There are several types of these. Im using the one for a late 90's truck (vortec?), which uses a simple rubber grommet rather then NPT threads. This makes it easier to mount in plastic.

 

I've also noticed this in both of my 91's.

It has basically disappeared with the Formula running 30# injectors and 48 psi with appropriate tuning.

It is quite apparent on the GTA which is 98% stock still.

I'm going to play around a bit and see what happens.

It is 100% a heat soak type problem, but mine is very apparent when higher outside temps are hotter. Will only do it on hotstart heat soaked conditions.

later
Jeremy

 

Where is the stock location for the IAT? In the manifold?

 

The IAT on a stock TPI engine is in the lower side of the plenum, toward the center, and in front of the distributer.

 

Thats a bad place for it...

 

If you relocate the MAT/IAT on a 7730, make sure you adjust your MAT Delta Inverse table. The relocating of the sensor affects this table and it should to be adjusted.

 

I know it's not the correct way to fix the issue but I unchecked the box in TunerCat that says "Enable Inv. MAT Term Lookup Table" and the problem is 90% better. I will do the relocate and modify the table when time permits. Thanks.

Regards,
Gary Anderson

 

I posted a "correction table" a while back which is a "lot closer" than the stock table. It's not perfect, but it's pretty darn close. Closer than the stock table with the stock setup IMO.

Just do a search on "MAT Inverse Lookup" and you'll find it in one of my posts PLUS a lot of subsequent information by RBob (and others) that went and reviewed exactly how that table worked.

 

I worked perfectly for me!!! No matter what temperature my engine is at now, it's and anytime, all the time!!

Hands down, your MAT tables were one of the best mods I did to my cal

 

I changed my stock MAT to an open cage style one from a 90 Bonneville. Its a different connector but the calibration is the same. Seems to recover from heat soak faster.

 

Did they actually use a CTS sensor there for the IAT?

I can sort of see the logic for putting it in the plenum as its hard to estimate the plenums temperature and contribution using the MAT and CTS, but some of those sensors take forever and a day to reach steady state.

 

Yeah, I know they have the technology to at least cut that down to forever!

 

If I find junkyard parts sources(harness connectors) here in houston I will relocate mine here or at the worst in july when im home for three weeks. (might have sensor floating around here in the gta)

Might try disabling the bit as well.

Lack of funds hurts more than anything as usual lol

later
Jeremy

 

GrimReaper,

How did you go about "tuning" the MAT table?

 

Here are the stickies on MAT tables...

I had to reread these things multiple times before it got through my 1/2" thick skull...

https://www.thirdgen.org/forums/diy-...-question.html

https://www.thirdgen.org/techbb2/sho...postid=1474826

https://www.thirdgen.org/techbb2/sho...hreadid=265869

 

ULTM8Z,


Thanks. Actually read a couple of those previously. The RBob post is a keeper. I am using a ZR-1 IAT mounted in the radiator shroud. RBob's EBL uses a IAT/CTS Blend table with %CTS. Not sure how to interpret MAT table to that if it can be done at all. The 84 Xfire Vette has long intact tracts that are molded as part of the hood and lead to the air intake inlets in the air cleaner assembly. So I think there's plenty of time to transfer heat into the intake air. Consequently, what seems to have worked best is a max of 71% CTS to IAT blend at 255gms. It drops to 93% by the time I get to 0gms.