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Engine has mood swings

1990 trans am 305 no power sluggish surge hesitate

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15 replies to this topic

#1 Teslonelf

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  • Year: 1990
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  • Model: Firebird Trans Am
  • Engine: 305 TPI
  • Transmission: Manual 5-Speed

Posted A week ago

1990 Firebird Trans AM

305 TPI VIN F

5 Speed Manual Transmission

 

To start, this car was parked for 3 years before I started working on it. It was parked because (from what I've been told) it would die under any load and when starting it they had to hold RPMs at 2k for a few minutes before it would continue running on its own.

 

2 months before I got it running, I replaced the rotor and distributor cap, spark plugs and cables, air filter, serpentine belt, MAP sensor, battery and ignition coil, which were parts that had been listed by a mechanic friend of the owner as needing replaced. Since the owner purchased the parts and wanted them in, I didn't bother checking the old parts to see if they were actually bad, but in hindsight I probably should have. Added fuel and engine treatment to a fresh tank of fuel for the first several tanks and noticed significant improvement over that time, however the old problems seemed to die off and new issues cropped up.

 

The owner wanted to try to see if she would start and run and after several attempts, she did, but had the same issues.

 

I replaced the fuel filter and now the car runs and drives. The problems are intermittent and each time she's driven she has a different list of symptoms, which can also change while driving.

 

Symptoms List #1

- Engine will not rev high in neutral, or will not rev quickly (this can be as low as a 2k limit or as high as 4500)

- Under load, car will lurch then sag

- Idle is high at temp (between 900 and 1500)

 

Symptoms List #2

- Engine will rev fine in neutral but will surge and hesitate under load

- Shift Indicator will always indicate "Up" until car is in 5th gear

- Idle is high at temp (between 900 and 1200)

 

Symptoms List #3

- Car has no issues in 2nd, 3rd, or 4th, but 1st will surge and hesitate, and 5th has no power until car is in excess of 53mph (around 1300-1400 rpms in 5th)

- Idle is high at temp (between 900 and 1500)

 

Symptoms List #4 (Very rare)

- No issues whatsoever

 

In most of the above cases, fluttering the accelerator will allow the car to accelerate at an almost normal rate. The Check Engine Light works, but will only turn on once in a great while when at highway speed and will only remain on for 5-15 seconds.

 

Now, I am not a mechanic, but I have lots of experience working on cars (mostly Dodges). There are several parts that I think may be the culprit but the owner and I do not have the time or money to just replace everything. I'm hoping to get a list of most likely's or things that I can or should check.

 

Also, I do have the shop manual (the factory shop manual), so if you can point me to specific sections or pages for things it would be really helpful.


Edited by Teslonelf, A week ago.


#2 Injuneer

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  • Year: 1994
  • Make: Pontiac
  • Model: Firebird Formula
  • Engine: 381ci LT1 stroker
  • Transmission: TH400 + GV O/D

Posted A week ago

Have you scanned the ECM for codes? Have you checked the fuel pressure at idle, and with the engine under load?

If the check engine light comes on, the code that turned it on is stored and can be retrieved by shorting a couple pins in the diagnostic connector, and counting flashes on the check engine light.

Does it still have a catalytic converter? Any chance it is partially plugged?

1994 FIREBIRD FORMULA

FRED

381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC M48 Pro engine management - 8 LS1 coils - 58mm TB - 74# injectors - 300-shot dry nitrous - TH400 - Gear Vendors 0.78:1 O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

 

gallery_709_698_32777.jpg


#3 Teslonelf

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  • Model: Firebird Trans Am
  • Engine: 305 TPI
  • Transmission: Manual 5-Speed

Posted A week ago

Have you scanned the ECM for codes? Have you checked the fuel pressure at idle, and with the engine under load?

If the check engine light comes on, the code that turned it on is stored and can be retrieved by shorting a couple pins in the diagnostic connector, and counting flashes on the check engine light.

Does it still have a catalytic converter? Any chance it is partially plugged?

I totally have not checked the trouble codes. In my experience (again, mostly Dodges) if the Check Engine Light isn't on, there are no trouble codes. I'll check that tonight and see what's up.

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#4 Teslonelf

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  • Model: Firebird Trans Am
  • Engine: 305 TPI
  • Transmission: Manual 5-Speed

Posted A week ago

I just checked the trouble codes and it's just giving me code 12 (no trouble codes). I recently had unplugged the battery to work on other things, though, so I'll have to check again after I see it flash on on the highway again.

Checking fuel pressure and the cat will take some time since I don't have the tools on location.

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#5 Injuneer

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  • Model: Firebird Formula
  • Engine: 381ci LT1 stroker
  • Transmission: TH400 + GV O/D

Posted A week ago

The "no power" until 1,300 RPM in 5th gear doesn't seem like a problem. 5th is the O/D gear, 0.63:1 I think. Engine's not going to have much torque at low RPM in O/D. Just for freeway cruising. Drop it to 4th or even 3rd to accelerate. Depends what the rear axle ratio is also.

1994 FIREBIRD FORMULA

FRED

381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC M48 Pro engine management - 8 LS1 coils - 58mm TB - 74# injectors - 300-shot dry nitrous - TH400 - Gear Vendors 0.78:1 O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

 

gallery_709_698_32777.jpg


#6 Teslonelf

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  • Engine: 305 TPI
  • Transmission: Manual 5-Speed

Posted A week ago

When the car is driving normally, you can enter and accelerate in 5th starting at 45mph (roughly 1000-1100rpm). At times, with these issues, the car won't even be able to maintain speed until reaching 53mph.

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#7 sea dog

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Posted A week ago

You need to find an obd 1 code scanner with real time sensor data capability. You need to run data scans of car. Start with engine cold and check data the whole time car is running. Best to have an assistant drive while you check data.

 

Compare data when car is running well to data when symptoms appear. You might be able to find the source of your trouble.

 

Since I don't know your skill level, if you have trouble understanding sensor data, I can send you a pdf called, "scanner readings and what they mean". It was written by Injuneer. If you want it PM me with your e-mail address

 

If you don't have an obd 1 scanner, you can use the free scanning program called 'freescan" to scan your 90. If I remember correctly, it will scan and log data for later playback. You should be able to do a web search and find it. If you can't, send e-mail address and I'll send you a copy of the program.

 

Program is pc based. You will need an aldl to usb port 12 pin cable to use freescan. You can get athttp://www.obd2allinone.com/aldl.asp


Edited by sea dog, A week ago.


#8 Teslonelf

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  • Engine: 305 TPI
  • Transmission: Manual 5-Speed

Posted 5 days ago

While I was checking on the O2 sensor, I was checking the passenger side to see if there was another one (apparently this car has one on the driver's side only) and I saw a connector that looked hinky. I'm not sure what it is. After taking the picture, I tapped it and the connector fell out and the plastic sheath shattered. Any ideas? 8d561fee4c86b255e050b2a07e8dd72e.jpg

#9 Injuneer

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  • Year: 1994
  • Make: Pontiac
  • Model: Firebird Formula
  • Engine: 381ci LT1 stroker
  • Transmission: TH400 + GV O/D

Posted 5 days ago

Is that screwed into the lower passenger side of the block? Looks like might be a knock sensor.

1994 FIREBIRD FORMULA

FRED

381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC M48 Pro engine management - 8 LS1 coils - 58mm TB - 74# injectors - 300-shot dry nitrous - TH400 - Gear Vendors 0.78:1 O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

 

gallery_709_698_32777.jpg


#10 Teslonelf

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  • Engine: 305 TPI
  • Transmission: Manual 5-Speed

Posted 5 days ago

Yeah, that's where it's at. Definitely want to get a new connector for it. And if my research is accurate, that might be a contributing factor. From what I'm reading, if the knock sensor isn't connected or is bad, the ECM changes to a "limp home mode".

#11 Injuneer

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  • Engine: 381ci LT1 stroker
  • Transmission: TH400 + GV O/D

Posted 5 days ago

Loss of signal the PCM retards the ignition timing. Would set a code. Think it's 43. At least if it's like the later TPI'S.

1994 FIREBIRD FORMULA

FRED

381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC M48 Pro engine management - 8 LS1 coils - 58mm TB - 74# injectors - 300-shot dry nitrous - TH400 - Gear Vendors 0.78:1 O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

 

gallery_709_698_32777.jpg


#12 Injuneer

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  • Year: 1994
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  • Model: Firebird Formula
  • Engine: 381ci LT1 stroker
  • Transmission: TH400 + GV O/D

Posted 5 days ago

Could also be a fan temp switch. Typically has a gray wire. The knock sensor has a blue wire.

http://www.partsgeek...&ad=47433966132

1994 FIREBIRD FORMULA

FRED

381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC M48 Pro engine management - 8 LS1 coils - 58mm TB - 74# injectors - 300-shot dry nitrous - TH400 - Gear Vendors 0.78:1 O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

 

gallery_709_698_32777.jpg


#13 Teslonelf

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  • Year: 1990
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  • Model: Firebird Trans Am
  • Engine: 305 TPI
  • Transmission: Manual 5-Speed

Posted 4 days ago

Replaced the O2 sensor and the pigtail for the knock sensor. The old O2 sensor looks like it's probably the original. Reset the ECM and drove the car a bit. Works much better and no trouble codes. Still hesitant and occasionally have to flutter the gas to get moving or accelerate.

Tomorrow, I'm pulling the plugs and double checking the gaps, and verifying that they're not cruddy or gunked up. If that all checks out, I'll be pulling the cat.

#14 Teslonelf

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  • Engine: 305 TPI
  • Transmission: Manual 5-Speed

Posted 3 days ago

Seems the spark plugs were *not* gapped appropriately by the guy that sold them to me. They were all between 0.04 and 0.045. Now at 0.035 and she's running much better. Still have a little trouble going from a stop at first and accelerating at all in 5th. Checking fuel pressure on Monday when my gauge gets returned.

#15 Teslonelf

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  • Engine: 305 TPI
  • Transmission: Manual 5-Speed

Posted A day ago

Fuel pressure is at 10 with the ignition on and the engine off. After starting the car, it went up to 22. It's supposed to be at 47, right?

There's no access hatch in the back, so what tools do I need to replace the fuel pump myself?

#16 Injuneer

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  • Year: 1994
  • Make: Pontiac
  • Model: Firebird Formula
  • Engine: 381ci LT1 stroker
  • Transmission: TH400 + GV O/D

Posted A day ago

At idle, no vacuum compensation line attached to fuel pressure regulator, 43.5 PSI. Anything between 41-47 PSI is OK. Reconnect vacuum line and pressure should drop about 8 PSI with a stock cam, to mid 30's. Less drop w/ a more aggressive cam.

3rd Gen is basically the same as the 4th Gen. To drop the tank "by the book", drop exhaust, drop rear axle assembly, wrestle tank out. Many 4th Gen folks cut a hatch in the package shelf to access the fuel pump assembly. Below is a link to the 4th Gen procedure, but you would have to verify details/dimensions w/ a 3rd Gen owner.

If you cut a hatch, no square corners. Radius corners to avoid stress risers. When complete cut a patch panel that overlaps the opening by about an inch all around. Apply body sealer fully around the opening, and install cover w/ multiple short sheet metal screws or pop rivets. You don't want fuel leaking into the interior if there is an accident w/ a roll over. Since this appears to be a customer's car you are working on, it would be up to the owner whether the hatch method is acceptable. And whoever does the work assumes all risk.

https://www.camaroz2...4th-gen-619463/

1994 FIREBIRD FORMULA

FRED

381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC M48 Pro engine management - 8 LS1 coils - 58mm TB - 74# injectors - 300-shot dry nitrous - TH400 - Gear Vendors 0.78:1 O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

 

gallery_709_698_32777.jpg






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