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1995 Firebird Trans Am, 5.7L 350ci, 4l60e.
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Discussion Starter · #1 ·
So I just got my motor swap running but having a issue with the TPS I'm guessing because it's idling low and stalling or the idle will rise real high. I did upgrade from a stock TB to a BBK 58mm TB so I heard that can causes issues.

Also my gas pedal is not working correctly when I hit the gas it only opens the TB like a 1/4th maybe 1/3rd of the way.

What trouble shooting steps do I need to take?

If I'm leaving out any important information let me know and I'll reply.

Thanks, Steven.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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What modifications justified the 58mm throttle body? Heads? Cam? Was the intake manifold bored to match the throttle body?

What is the idle RPM, fully warmed up (above 176-degF)? Stock programming with the 4L60E is 550 RPM in gear, 650 RPM in neutral. When does the idle "rise real high"? What RPM? Is this like a surge, with the RPM swinging up and down? If seriously modified, who tuned the PCM?

The throttle open vs. pedal movement has to be a cable/linkage issue. Or carpet under the pedal.....??? Are both the pedal cable and the cruise control cable connected? Possible the TPS is not indexed correctly to the throttle shaft?

Do you have access to a scan tool/software that will log PCM data, or show it real time? It would be good to know how the IAC is responding to the RPNM changes. Also be useful to know the throttle position sensor voltage.
 

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1995 Firebird Trans Am, 5.7L 350ci, 4l60e.
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91 Posts
Discussion Starter · #3 ·
I upgraded the TB because I ported the entire intake and yes it matches the TB. I also ported the heads and it has a 07-503-8 CompCams Cam.

The idle Rpm is like 900-1000 rpm but it will only stabilize if I give it some gas otherwise it will surge up and down then stall. I haven't checked the RPM warmed up just on start up. On about one or two start ups the idle was slowly rising real high like 3000 rpm before I shut it off it wasn't surging just slowly raising., it doesn't always do this. When it is idling at around 1000rpm If I try to hit the gas it will shoot real high as well like around 3-4k RPM.

It has not been tuned yet but I plan to get that done. I was planning on going through Solomon from lt1pcmtuning. I'm also ordering a cable so that I can log information myself.

I know that the power wire to TPS is supplying 5v but I haven't tested the TPS voltage what is the range it should be in when throttles closed?

I will check the cable linkage. No carpet in the way.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Get it tuned now. I would strongly advise that you consider someone like Moe Bailey. I can provide contact info. I won't comment on your choice on a pubic forum, but you san send me a private message. The cam is not that large, but the increased overlap may cause issues with idle and low RPM surge.

Unless the engine is making at least 550-600+ HP at the flywheel, the 58mmTB is total overkill. I had a Holley 58mm on my 500 HP stroker and the shop was able to show me, on the engine dyno, that the engine got all the air it needed with the 58mm TB only about 77% open. A 52mm TB would have supplied all the air it needed. They still laugh at me for that one.

Closed throttle voltage can be anything between 0.30-0.90 volts without setting a code. But to play is safe, set it between 0.50-0.70 volts. The factory TPS is not adjustable, so you may have to slot the holes so the body of the sensor can be rotated. The blades should be gradually opened, watching voltage. Should increase smoothly, no spikes, no dropouts, WOT voltage should be about 4 volts higher than closed throttle voltage. Check to make sure the spring in the sensor had rotated all the way back to the stop (closed throttle position), and the the flats of the shaft that it slips over are in the closed position. Conceivably it could be off by 180-deg.
 
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Discussion Starter · #5 · (Edited)
Thx Injuneer you're always a great help. I got in contact with Moe he is gonna be doing the tune.

As far as the TB goes does overkill mean that it's going to hurt peak performance or overkill in the sense that I'm not getting the most out of the TB?

I don't really want to replace it unless a 52mm would give better performance.

Also I checked the TPS signal voltage and it was between 0.15-0.17 so would that cause the issues described above?
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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“Overkill” means not required, so it was money you could have spent elsewhere. But being oversize can cause issues with the automatic transmission shifting, and the PCM’s response to throttle movement. When the PCM sees the throttle opening, the faster it opens, the more fuel it adds to prevent the engine from leaning out as the air flow increases. With the 58mm throttle blades small movement producing a larger amount of air flow than the PCM is expecting, it may lean the engine out and cause a stumble. I suspect the tuner can compensate somewhat for both the shifting and the lean out. But I'm not a “tuner”.

With the TPS closed throttle volts below ~0.25 volts (varies from year to year) the PCM should set a code. That may cause the PCM to use some other criterion for fueling and shifting.
 

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1995 Firebird Trans Am, 5.7L 350ci, 4l60e.
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91 Posts
Discussion Starter · #7 · (Edited)
I thought that was the purpose of the Mass Air Flow sensor to calculate the air flow for the PCM so its not confused?

As far as the shifting is concerned do you think its worth doing a vacuum modulated modification for the 4l60e like TransGo?

I just bought a ALDL cable so i can check codes.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Yes, the MAF sensor directly measures air mass flow into the engine, as an alternative to the speed-density method of calculating mass air flow. The MAF sensor can't react fast enough to the sudden increase in flow due to the throttle blades opening rapidly. The added fuel based on rate of increasing throttle position replicates the accelerator pump on a carburetor.

Have no idea if the Transgo 46-MOD kit would be effective. I thought that was a heavy duty pressure control for applications that did not have tuneable pressure control.
 
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Discussion Starter · #9 ·
Do you think Its worth upgrading my fuel injectors over the stock 24s or not necessary? If so what size would you recommend?
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Always difficult to select injector size without knowing the engine's expected HP at the flywheel. The unknown here is the heads. Did you port them yourself, or did a shop do it? Either way, do you have flow numbers?

My guideline is the multiply flywheel HP by 0.07 to get a reasonable injector size (normally aspirated only). Stock 24’s are OK for 340-350 flywheel HP. At 400 HP x 0.07 = 28 #/HR. 500 HP x 0.07 = 35 #/HR.

Too small an injector limits power and can lead to detonation and engine damage. Fortunately, a larger than necessary injector is not a big problem (within reason), as long as the PCM is programmed with the correct injector flow rate, along with the correct offsets.
 
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Discussion Starter · #11 ·
LE1 heads from Elliott with ported Intake from him as well and all the other bolt ons but no power adders. I'd hope it's atleast 400. I was thinking of going with 28 or 30# I wanted to change them before I get the tune if I need to.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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With LE1 heads and a cam similar to yours, he estimates 360-380 rwHP. Through a 4L60E that would correspond to about 415 - 440 flywheel HP. 440 x .07 = 30.8 #/HR. I would recommend at least a 32 #/HR injector. The tune has to have the specifics of the injectors you buy, including the offsets. The tuner wi know the offsets if he has seen the soecifc injectors before.

Consider this source. I like the Bosch 3 injectors. If they don't have a 32, look at a 36 #/HR injector.

 
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Discussion Starter · #13 ·
I was actually just looking at those but im not gonna be able to afford those right now. I know its not ideal but should i just stick with the stock 24s or buy used injectors or new from a china ebay brand? I can probably buy those Bosch within the year.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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DO NOT BUY a set of 8 CHINESE TRASH injectors. You have invested a lot of $$$$ on your build. Don’t ruin it…. even if prices as low as $35 for a complete set seems irresistible. Most of the eBay “Bosch” injectors are counterfeit.
 
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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Another reliable source:

 

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1995 Firebird Trans Am, 5.7L 350ci, 4l60e.
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Discussion Starter · #16 ·
What do you think of the FIC 32 LB BOSCH DESIGN III SET OF 8... its only 250$ for those they are rebuilt from that first site u posted but I can prob afford those?
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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FIC is honest. They are respected. Their tech videos are impressive, filtering out fact from fiction. I wouldn’t hesitate to trust their rebuilds. I also feel Bosch injectors are among the best in mass production.

I got my Bosch (not 3’s) low impedance 64 #/HR injectors used, from a friend. They weren’t big enough as he approached 1,000 HP with his supercharged 383ci LT1. That was 22 years ago. I ran them at 58 PSI to up the flow to 74 #/HR. Just sold the Formula, and the buyer did nothing but have them professionally cleaned, since the car sat several years unstarted, with a blend of 94 octane Sunoco pump gas, and VP Fuels C16 highly leaded, 116 octane blended 3:1 with their “Air Race” aircraft racing fuel.

Video of results in post #2:

 

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1995 Firebird Trans Am, 5.7L 350ci, 4l60e.
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Discussion Starter · #18 ·
I actually was actually looking at that yesterday! Nice looking car and love the sound. My Firebird is actually originally that same color.

Think I will get those then now I can get the car tuned because Moe sent me a spec sheet so i wanna make sure I get it all together now.
 

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1995 Firebird Trans Am, 5.7L 350ci, 4l60e.
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Discussion Starter · #19 ·
You have any idea what my compression ratio would be? I put on .040'' thickness MLS head gaskets im not sure what to tell Moe i dont know how to calculate it.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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There are online compression calculators. You need:

  • cylinder bore
  • stroke
  • head combustion chamber cc's
  • (compressed) gasket thickness
  • deck height
  • piston valve relief cc's

This one is easy to use:

 
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