Thread: Hello from Cali - hot start issue

Hey guys,

I picked up 2002 GTP a few months ago. Started modding it, but now it's getting a little more serious, so here I am. It's a 40th anniversary with 58k miles (got it at 55k!). The two previous owners were both old ladies, so it's going to have a long life.

Current mods:

Downpipe
3.6" pulley
CAI
Autolite 104s
180 tstat
zzp 1.0 tune :/
Solid engine and trans mounts

Mods to come soon:

Front plog
Ported S/C
Ported LIM
Northstar TB

Eventually:

FSIC


I do have a hot start issue if anyone feels compelled to help out. Cold starts are always fine. Once that car has been sitting for ~5 mins or more after driving, starting it can be a chore. The longer it sits, the harder it gets to start. Once it's cold again, it fires right up no problem. Once it's been parked for 5+ mins, what will happen is you start it and it will go up to around 1200 RPM, then it will just fall flat and stall without putting up much of a fight. You have to give it gas for a few seconds to keep it running, and after 5 seconds or so it will keep itself running. If it's been sitting for 30-60 mins it gets really hard to keep it running for the first few seconds. Sometimes some black clouds will shoot out when doing this - it seems to be dumping fuel. Most of the time the idle will bounce around for 15 seconds or maybe more, fluctuating from 500RPM - 1500RPM, and eventually finding it's home around 800. After this hot start condition is hit, accelerating from a stop will bog for a split second when you first tip the throttle in. After that and you are moving, it drives fine, even under full throttle. Also something to note - stft are pretty negative at idle sometimes, even up to -20. While driving, it seems to be in the normal range of +/- 8.

What I've tried so far:

Cleaning MAF
Cleaning TB
Replacing IACV (this helped with starts but didn't fix the issue)
Tested Fuel Pressure Regulator. Took vacuum line off and let car idle for ~3 mins, no fuel came out.
Unplugged MAF - this seemed to help with the issue although I need to do some more testing

What happens if you wait for it to be a hot start and just prime the fuel system once or twice, then pull the vacuum line off the regulator. It's better to check it when the car is off and see if fuel dribbles out then.

I'd want it hooked up to a scanner personally, that way you could watch MAF, MAP values, see what they are saying. I've seen faulty IAT sensor cause this too because if they read -40 all the time, that can cause flooding of fuel.


Just a side note, you do not need to upgrade to a northstar throttle body. You are no where near close to outflowing the stock throttle body or MAF sensor yet.

Well today I ended up confirming that the MAF is bad. The hot start issue doesn't happen if I unplug the MAF. I was also getting really rich idle with the MAF plugged in, stft was pegged at -25 at one point. I still don't really get why it only affects the car on hot start though... Anyways I think the MAF is aftermarket since it looks pretty new. I'm guessing that's the problem.

As for the Northstar - I'm getting the inlet in the s/c ported along with the outlet, and plan on getting a FSIC in the future. I've decided to go with the Northstar so it will be basically prepped for good airflow when I get the IC. And now that I've confirmed my MAF is bad it helps the decision.

Welp if that the way you go, toss that ZZP tune in the trash because it's worthless if you go the Northstar and different MAF route.

ZZP will never be able to send you anything worth a salt because they don't have a clue how much air the engine needs. It will need a complete retune of the MAF airflow table from scratch along with WOT tuning and so on. Just thought I'd say that because many don't know what really has to be done.

Originally Posted by Fivefingerdeathpunch

Welp if that the way you go, toss that ZZP tune in the trash because it's worthless if you go the Northstar and different MAF route.

ZZP will never be able to send you anything worth a salt because they don't have a clue how much air the engine needs. It will need a complete retune of the MAF airflow table from scratch along with WOT tuning and so on. Just thought I'd say that because many don't know what really has to be done.

Do you think I should go for the high velocity tb instead of the Northstar? At what point do you start outflowing the stock tb?

Somewhere around the 3.2-3.3 pulley range you be right around the max airflow of the MAF sensor. You could either install a larger TB/MAF combo or install a mini AFC to get around the MAF limit.

Even with the high volocity throttle body it will need a custom tune because the port size of the throttle body is different and the MAF is right in the center of it. Which means different amounts of airflow at any given time around the MAF and the stock calibration will be off.

Originally Posted by Fivefingerdeathpunch

Somewhere around the 3.2-3.3 pulley range you be right around the max airflow of the MAF sensor. You could either install a larger TB/MAF combo or install a mini AFC to get around the MAF limit.

Even with the high volocity throttle body it will need a custom tune because the port size of the throttle body is different and the MAF is right in the center of it. Which means different amounts of airflow at any given time around the MAF and the stock calibration will be off.

What's your opinion on porting the inlet of the s/c? Is it even worth it? Also, IF I got the inlet ported but didn't get a bigger TB, would that cause any problems?

The only reason I was planning on getting a new TB is because of the s/c inlet porting (and IC in the future).

Rather than porting your M90 Gen 3 and LIM, swap it to an M90 Gen V and LIM from an 04+, then have that ported.

And instead of a plog, get headers. They don't have to be ZZP headers, Ebay with some modification works fine. Use the search feature here, or google for more info on that.

Once you're more comfortable with where the car is, or before you change injectors or MAF, get DHP or HPTuners and learn to scan an tune yourself.

Originally Posted by Explicit_Spade

Rather than porting your M90 Gen 3 and LIM, swap it to an M90 Gen V and LIM from an 04+, then have that ported.

And instead of a plog, get headers. They don't have to be ZZP headers, Ebay with some modification works fine. Use the search feature here, or google for more info on that.

Once you're more comfortable with where the car is, or before you change injectors or MAF, get DHP or HPTuners and learn to scan an tune yourself.

Had I done more research before ordering some of the first mods, I probably would have ended up with headers. I really didn't think I would want to mod much further than just a dp/pulley/tune since it's just my daily, but I was wrong. I like the idea of scooting past people in a purple pontiac.

At this point I already have the dp on and the front plog would be an easy addition without much work. I'm not exactly mechanically adept, so headers aren't something I'm too excited about tackling. Gen5 is definitely something I am considering.

As for tuning - that's definitely something I'm comfortable with. I have access to HPTuners and I used to do my own tuning on my other car, even scaling MAF for larger intake. I just have to learn this platform. It's probably MUCH easier than what I'm used to.

Thanks for the input. I have a lot to learn.