L98 starts and dies after new intake
#1
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Car: 1988 Trans Am GTA
Engine: 5.7 L98
Transmission: 700r4
Axle/Gears: Borg Warner 9 bolt 3.27
L98 starts and dies after new intake
I’m starting to get frustrated after finally getting my car back together after installing a new big mouth intake, AS&M runners, and ported plenum. The car starts and wants to immediately die if gas is not given right away and if it dies will only start with hitting gas pedal. Also idle surges after letting off gas when idling, like hitting 450 and going back 850-900. My buddy and I almost think it’s off a tooth but we double checked it and timed it to 8 degrees BTDC. I am thinking the idle air needs to be adjusted to accommodate the aftermarket parts. I also looked at the L98 start up sequence and according to that it’s either vacuum leak, intake leak or idle air mixture on TB. The idle has never been touched either as evidenced by the factory plug in the TB.
After driving 30 miles the idle has improved with the re-learn but still does not want to start up correctly. The FPR is 2 years old as well as the Bosch 3 injectors and no check engine light. I also double checked TPS voltage and did the IAC reset. Fuel pressure is at spec around 43-44 with fpr line disconnected.
When it was idling with aldl droid
After driving 30 miles the idle has improved with the re-learn but still does not want to start up correctly. The FPR is 2 years old as well as the Bosch 3 injectors and no check engine light. I also double checked TPS voltage and did the IAC reset. Fuel pressure is at spec around 43-44 with fpr line disconnected.
When it was idling with aldl droid
#2
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Car: 92 Firebird, 77 Trans Am SE, 86 Z28
Engine: 5.7 HSR, T/A 6.6, empty
Transmission: T-5, TH350, T-5
Axle/Gears: 3.08 posi, 3.23 posi, 3.23
Re: L98 starts and dies after new intake
Get or make a cheap smoke machine and check for vacuum leaks. Especially since it says lean.
I find that brake cleaner checks aren't reliable at all and when you can build a cheap one for probably less that $30 it's hard to beat. Last I looked, you could get a built one that uses smoke oil on Amazon for $80 which still isn't bad.
I find that brake cleaner checks aren't reliable at all and when you can build a cheap one for probably less that $30 it's hard to beat. Last I looked, you could get a built one that uses smoke oil on Amazon for $80 which still isn't bad.
Last edited by aliceempire; 04-14-2021 at 07:41 PM.
#3
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Car: 1988 Trans Am GTA
Engine: 5.7 L98
Transmission: 700r4
Axle/Gears: Borg Warner 9 bolt 3.27
Re: L98 starts and dies after new intake
Get or make a cheap smoke machine and check for vacuum leaks. Especially since it says lean.
I find that brake cleaner checks aren't reliable at all and when you can build a cheap one for probably less that $30 it's hard to beat. Last I looked, you could get a built one that uses smoke oil on Amazon for $80 which still isn't bad.
I find that brake cleaner checks aren't reliable at all and when you can build a cheap one for probably less that $30 it's hard to beat. Last I looked, you could get a built one that uses smoke oil on Amazon for $80 which still isn't bad.
#4
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Car: 1988 Trans Am GTA
Engine: 5.7 L98
Transmission: 700r4
Axle/Gears: Borg Warner 9 bolt 3.27
Re: L98 starts and dies after new intake
Also the car can be driven around, even starting and stopping frequently, without stalling to the point I thought it was good and then stopped and restarted it but didn’t want to stay running and had to hit the gas while turning the key to get it to start up.
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Car: 1988 Trans Am GTA
Engine: 5.7 L98
Transmission: 700r4
Axle/Gears: Borg Warner 9 bolt 3.27
Re: L98 starts and dies after new intake
I wound up changing the idle air mixture a touch following the instructions in the how to section and now the car starts and doesn’t die and no idle issue at all. I’m sure it’s just masking what the real issue is but at least now I can drive it without having to constantly re start etc. I’m pretty sure there are small vacuum leaks around the runner to base and runner to plenum.
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Car: 1988 Trans Am GTA
Engine: 5.7 L98
Transmission: 700r4
Axle/Gears: Borg Warner 9 bolt 3.27
Re: L98 starts and dies after new intake
Took another crack at it and found a vacuum leak around the driver side front runner to base gasket with my TB cleaner, I loosened everything up on that side and re-torqued it all. Hopefully that will help the leak, they are new gaskets so I might have just needed to reseat it in the right place.
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#8
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Car: 1988 Trans Am GTA
Engine: 5.7 L98
Transmission: 700r4
Axle/Gears: Borg Warner 9 bolt 3.27
Re: L98 starts and dies after new intake
Still getting wonky numbers on aldldroid but I busted out my vacuum gauge and I’m pulling nearly 20 inches of vacuum...18 at idle to be exact at the FPR port.
#10
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Re: L98 starts and dies after new intake
Maybe TPI just sucks a lot
Glad to hear you're getting it figured out though, I'm putting my rails and runners back on soon.
Glad to hear you're getting it figured out though, I'm putting my rails and runners back on soon.
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#11
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Car: 1988 Trans Am GTA
Engine: 5.7 L98
Transmission: 700r4
Axle/Gears: Borg Warner 9 bolt 3.27
Re: L98 starts and dies after new intake
Lol it really does sometimes, but when it runs right it’s pretty good. At least the car can be ran right now but I’m running out of ideas.
#12
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Car: 1988 Trans Am GTA
Engine: 5.7 L98
Transmission: 700r4
Axle/Gears: Borg Warner 9 bolt 3.27
Re: L98 starts and dies after new intake
Tried a cold start this morning before work just to see how it behaves and it started but immediately started hunting for idle from around 500-800 rpms...so back at square one. I didn't have the time to let it get warm and see if it smoothed out but considering how well it ran when warm the temperature seems to play a key part in this. I might order up a set of Mr Gasket 146 gaskets and replace that one runner to base gasket just to eliminate it as a possible cause, at least its on the driver side.
Last edited by Reddragon88gta; 04-16-2021 at 08:40 AM.
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Car: 1988 Pontiac Firebird Formula
Engine: 5.7 TPI
Transmission: 700R4
Re: L98 starts and dies after new intake
May be a cold vacuum leak, that seals itself up as the engine warms, and expands.
#14
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Car: 1988 Trans Am GTA
Engine: 5.7 L98
Transmission: 700r4
Axle/Gears: Borg Warner 9 bolt 3.27
Re: L98 starts and dies after new intake
I’m pretty sure your right, I’m going to order new gaskets and give it a shot. These cars are very touchy with vacuum issues.
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Car: 92 Firebird, 77 Trans Am SE, 86 Z28
Engine: 5.7 HSR, T/A 6.6, empty
Transmission: T-5, TH350, T-5
Axle/Gears: 3.08 posi, 3.23 posi, 3.23
Re: L98 starts and dies after new intake
If you smoke test it, you'll see where the leaks are. It might not necessarily be the tpi gaskets. If those gaskets are still in one piece then use some rtv. The Right Stuff is excellent for the MrGasket material. This is coming from first hand experience with a ported set that I had to modify each set of gaskets for the porting.
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Car: 1988 Trans Am GTA
Engine: 5.7 L98
Transmission: 700r4
Axle/Gears: Borg Warner 9 bolt 3.27
Re: L98 starts and dies after new intake
If you smoke test it, you'll see where the leaks are. It might not necessarily be the tpi gaskets. If those gaskets are still in one piece then use some rtv. The Right Stuff is excellent for the MrGasket material. This is coming from first hand experience with a ported set that I had to modify each set of gaskets for the porting.
#17
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Car: 92 Firebird, 77 Trans Am SE, 86 Z28
Engine: 5.7 HSR, T/A 6.6, empty
Transmission: T-5, TH350, T-5
Axle/Gears: 3.08 posi, 3.23 posi, 3.23
Re: L98 starts and dies after new intake
I stand by using brake cleaner or tb cleaner to find leaks is luck at best. I've sprayed areas that have smoke coming out of them and didn't have anything change in engine rpm. But it's neither here nor there.
I'd coat each side as thinly as possible. You don't want it squeezing gasket out of shape (that's a huge drawback to using rtv in gaskets) but you want to take up the imperfections in the surfaces. As said previously, aftermarket aluminum doesn't always meet the QC requirements of OE
I'd coat each side as thinly as possible. You don't want it squeezing gasket out of shape (that's a huge drawback to using rtv in gaskets) but you want to take up the imperfections in the surfaces. As said previously, aftermarket aluminum doesn't always meet the QC requirements of OE
#18
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Car: 1988 Trans Am GTA
Engine: 5.7 L98
Transmission: 700r4
Axle/Gears: Borg Warner 9 bolt 3.27
Re: L98 starts and dies after new intake
Got the gasket resealed last night and tried it today, vacuum leak appears to be gone. Still starts and wants to die immediately and not idle unless the idle screw is pushed out some. I do have some new live data once I got it to idle. It holds nearly 18 inch of vacuum at idle. Smells super rich
#19
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Car: 1989-92 FORMULA350 305 92 Hawkclone
Engine: 4++,350 & 305 CIs
Transmission: 700R4 4800 vig 18th700R4 t56 ZF6 T5
Axle/Gears: 3.70 9"ford alum chunk,dana44,9bolt
Re: L98 starts and dies after new intake
Do you have a good driver and pass side picture of the engine ? Have a few questions that are answered easier with pictures.
Also, I've noticed several after market lower runner flanges are larger than the machined area on the intakes and have to be ground on to fit.
Also, I've noticed several after market lower runner flanges are larger than the machined area on the intakes and have to be ground on to fit.
Last edited by TTOP350; 04-19-2021 at 07:03 AM.
#21
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Car: 1989-92 FORMULA350 305 92 Hawkclone
Engine: 4++,350 & 305 CIs
Transmission: 700R4 4800 vig 18th700R4 t56 ZF6 T5
Axle/Gears: 3.70 9"ford alum chunk,dana44,9bolt
Re: L98 starts and dies after new intake
Hard to see in the picture but does your driverside have the small tube between runners? (like the small tube on the pass side)
#22
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Car: 1988 Trans Am GTA
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Re: L98 starts and dies after new intake
Sorry the picture was lousy, it does and it’s blocked off by AS&M because I went with 89’ up runners because I had previously disabled the cold start injector in both the prom and on the rail.
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Car: 1988 Trans Am GTA
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Re: L98 starts and dies after new intake
I think I know what is wrong, the plenum I have is from an 85’, my throttle body is the original 88’ and with the 89’ up runners something is not playing together. Am I right to assume I need an 89 up throttle body???
#26
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Car: 1988 Trans Am GTA
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Re: L98 starts and dies after new intake
Or can I pull that driver side runner and unclog that small tube??
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Car: 1988 Trans Am GTA
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Re: L98 starts and dies after new intake
This is what it looked like when I got it before I cleaned it up and ported.
I received a PM about possibly drilling a hole in my 88’ throttle body to correct this issue as well??
Driver side
I received a PM about possibly drilling a hole in my 88’ throttle body to correct this issue as well??
Driver side
#30
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Car: 1988 Trans Am GTA
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Re: L98 starts and dies after new intake
Will do, what I was talking about with the TB was with the 89’ and up runners having that tube on the driver side blocked off on both ends would that effect my 85-88 plenum and throttle body? I was told when I ordered them they would work, I clearly explained my set up too to AS&M.
#31
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Car: 1989-92 FORMULA350 305 92 Hawkclone
Engine: 4++,350 & 305 CIs
Transmission: 700R4 4800 vig 18th700R4 t56 ZF6 T5
Axle/Gears: 3.70 9"ford alum chunk,dana44,9bolt
Re: L98 starts and dies after new intake
That tube sends the idle air to the lower intake, if it's blocked off by the later 89-92 runners or back of T-body with different parts, it will start and die. You can do some cutting on the triangle area ( front of the plenum) to just let the idle air in the plenum and it would be just fine.
Or port the correct plenum and it would be ok too
Or port the correct plenum and it would be ok too
#32
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Car: 1988 Trans Am GTA
Engine: 5.7 L98
Transmission: 700r4
Axle/Gears: Borg Warner 9 bolt 3.27
Re: L98 starts and dies after new intake
That tube sends the idle air to the lower intake, if it's blocked off by the later 89-92 runners or back of T-body with different parts, it will start and die. You can do some cutting on the triangle area ( front of the plenum) to just let the idle air in the plenum and it would be just fine.
Or port the correct plenum and it would be ok too
Or port the correct plenum and it would be ok too
#33
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Car: 1989-92 FORMULA350 305 92 Hawkclone
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Axle/Gears: 3.70 9"ford alum chunk,dana44,9bolt
Re: L98 starts and dies after new intake
This isn't mine but it's how I do them. For the most part it's mix and match at will then.
#34
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Car: 1988 Trans Am GTA
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Re: L98 starts and dies after new intake
Right there where I highlighted?
Any idea on how big a hole to drill? I see you that one is gone completely
#35
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Car: 1989-92 FORMULA350 305 92 Hawkclone
Engine: 4++,350 & 305 CIs
Transmission: 700R4 4800 vig 18th700R4 t56 ZF6 T5
Axle/Gears: 3.70 9"ford alum chunk,dana44,9bolt
Re: L98 starts and dies after new intake
Looks like you have a 85-88 t-body too, If you had a 89-92, you may have gotten away with it working.
3/8s to 1/2 ? Just make sure to clean the metal out really well.
3/8s to 1/2 ? Just make sure to clean the metal out really well.
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#36
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Car: 1988 Trans Am GTA
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Re: L98 starts and dies after new intake
The plenum and TB is 85-88, the guy at AS&M claimed this would work and I told him what my exact set up was with cold start delete etc...luckily this is an easy fix.
#39
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Car: 1988 Trans Am GTA
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Re: L98 starts and dies after new intake
I wound up going with the TB version of a fix by cutting out a groove in both sides of TB bores to IAC triangle, it worked great! Car idles right up and no more start and die. Thanks again for everyone who helped.
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#40
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Re: L98 starts and dies after new intake
Glad I found this. I believe I will encounter the same issue as I am changing my runners over to AS&M. I think I have the 89 up runners since I was planning a 9th injector delete... running EBL so no need any longer. Great info on this board and a few solutions presented. It is awesome when the OP comes back and posts the resolution instead of the thread just ending.
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#41
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Re: L98 starts and dies after new intake
I realize that you think it's fixed but your data indicates the MAF at idle is much too high and that's also reflected in the INT and BLM being buried in the negative fuel trim - it's pulling out as much fuel as it can because your MAF is reading almost 10 grams per second when it should be closer to 5-6 grams per second. MAF or wiring is boned.
GD
GD
#42
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Car: 1988 Trans Am GTA
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Re: L98 starts and dies after new intake
True, after seeing how rich it was the next day or so I put the Bosch MAF back and maf readings went back to 5 and BLM was almost right on 128 same as INT.
#45
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Re: L98 starts and dies after new intake
Unfortunately I did not but it looked similar to this and is what I used for guidance. You could also reference an 89’ and up throttle body. I made 2 grooves, one in each throttle bore connecting the IAC triangle shaped passage. I used a carbide cutting tool to do the job and it cut easily.
Here is a good thread that explains it as well.
https://www.thirdgen.org/forums/tpi/...8-vs-89-a.html
Here is a good thread that explains it as well.
https://www.thirdgen.org/forums/tpi/...8-vs-89-a.html
#46
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Car: 1989 Formula 350
Engine: Stealth Rammed 350
Transmission: 700R4
Axle/Gears: 3.45 9 bolt
Re: L98 starts and dies after new intake
Im having the same issue as well. Except I've got a Stealth Ram. When I start the car cold it doesn't want to idle. I need to help it out, then the IAC kicks in and starts hunting for idle. Usually ranging from 1500 down to 600. But ONLY when cold. If i start it warm. It fires right up and idles perfectly fine. Funny thing is, this only started After I took off the Granatelli Motorsports MAF and put a stock MAF on the car.
#47
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Re: L98 starts and dies after new intake
Im having the same issue as well. Except I've got a Stealth Ram. When I start the car cold it doesn't want to idle. I need to help it out, then the IAC kicks in and starts hunting for idle. Usually ranging from 1500 down to 600. But ONLY when cold. If i start it warm. It fires right up and idles perfectly fine. Funny thing is, this only started After I took off the Granatelli Motorsports MAF and put a stock MAF on the car.
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Car: 1989 Formula 350
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Re: L98 starts and dies after new intake
It had the aftermarket one on there and idled fine but didnt like to rev over 3K. Put a factory bosch one back on. Now it won't idle cold but it revs to 5K
#49
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Car: 1988 Trans Am GTA
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Re: L98 starts and dies after new intake
Do you have any way to get a data scan of what’s going on? That might help determine what’s going on or if one of the MAFs is getting messed up readings. I wish I could help more with that intake but have no experience with those.
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