Bogging after warm-up
10-25-2014, 03:04 PM
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Bogging after warm-up Good day to All,
A few years ago I decided to buy a 1994 Trans Am GT for my son so he could learn to work with his hands and on cars. He was 14 at the time. The vehicle I bought had 167K and I knew it was a fixer-upper.
Over those few years he/we have repaired everything from broken light sockets to installing new radio, speakers and an amp. Wheel hub, yup!, Rotors and pads, sure! Transmission mount, oil changes, the whole deal.
I was even able to get the car repainted.
The car ran "ok", but we noticed that everything on it seemed to be from 1994. Every part. Like nothing was ever changed.
When we got a chance for a free TB from BBK (lend them our car to test a new TB design and we can keep the TB), we jumped at that. The car and associated dyno run (which looked a lot like a stock market graph instead of a smooth power band) can be seen here:
Little did I know the car had an atrociously dirty MAF and a CAT that was for a 2" exhaust installed. This car had been necked down from 3" to 2", the cat installed and then necked back up to 3". I found these out and corrected both (cleaned the MAF and installed 3" Magnaflow CAT) when I tried to smog the car.
Then my son goes and cuts off the muffler to make it loud and proud.
Come to pass that my son blows a radiator hose and overheats the engine, blowing the head gasket. So we get the engine rebuilt. .030 over on the pistons, new timing chain, new gaskets, refurbished heads (he cracked both on the overheat), refurbished cam, new bearings, etc.
My problem is that after the rebuild, the car seems to drop 100-200 rpms and bog under light accel, but not all the time. It also smells a little like gas and get very bad gas mileage (10 or under with 65% freeway driving). From my time working on crotch rockets, it feels like an air/fuel problem.
Since everything on the car was 20 years old, I have slowly replaced almost every sensor on it knowing it is a censor driven car. Items replaced:
Throttle body: BBK
MAP Sensor: BWD
Evap Solenoid: BWD
Water Temp (Both): BWD
AIT Sensor: BWD
MAF: Delphi
02 sensors: AC Delco
Ignition Coil: BWD
Optispark: AC Delco/Delphi
Fuel Filter
EGR Valve: BWD
New Plugs (AC Delco) and Wires (7mm aftermarket)
Water Pump & Radiator
Items still left:
Throttle Position Sensor
Idle Air Sensor (bottom of TB)
Oil Pressure sensor
Fuel Pressure regulator
ICM
Knock Sensor
There is also a sensor down by the timing chain cover that has not been replaced.
ICM (but has been spaced off the block to prevent heat soak)
We have tested the fuel rail and it maintains 42psi with the key on and 36-7psi started and at idle, like the Chilton manuals says it should, so we don't suspect fuel pressure issues.
What we did find was most likely 20 year old fuel injectors with crappy/hard O-rings. We rigged up a test setup and found one injector that literally flowed fluid under pressure without any power and had some kind of hard precipitate in it that turned to a white ashy substance when it dried out. It also shot a **** stream when actuated. Replaced with BDW unit and replaced the O-rings on the remaining injectors. Test run last night was superior with a definite reduction is gas smell. This morning, drive to school was fine, but a quick run to the store a little later (restart after warm-up) and the same issues.
None of the other injectors spray like the new one. We tested it before install to see what a new/correct pattern should look like. They all spray varying levels of a stream surrounded by a cone of mist, whereas the new unit sprays a dense cone of mist. Also, my son said that two other injectors very slow-dripped under pressure but did not allow a flow of fluid through like the bad one. My understanding is that injectors should not allow any fuel through unless under demand.
So I am stuck, but maybe not: I am thinking of just replacing all injectors with 24# Accels. Philosophy being that everything is 20 years old and replacing a few could still lead to problems later. Is that the right approach and is it heading in the right direction? Am I missing something? The old pistons and valves had tons of crud and crap on them. Is it possible for an engine management system to slowly tune down as the engine system degrades over many years? Does it run bad at low throttle because my loud and proud son cut off the muffler and there is no back pressure? Did he jam the CAT with crap when he blew the head gasket?
A little bit of advice here could save me 4-5 Benjamins, and I am not too proud to say that I have reached the end of my knowledge base/skill set.
Thanks in Advance,
Sean
1994 Trans Am GT, 5.7L, BBK TB, no muffler, everything else stock.
A few years ago I decided to buy a 1994 Trans Am GT for my son so he could learn to work with his hands and on cars. He was 14 at the time. The vehicle I bought had 167K and I knew it was a fixer-upper.
Over those few years he/we have repaired everything from broken light sockets to installing new radio, speakers and an amp. Wheel hub, yup!, Rotors and pads, sure! Transmission mount, oil changes, the whole deal.
I was even able to get the car repainted.
The car ran "ok", but we noticed that everything on it seemed to be from 1994. Every part. Like nothing was ever changed.
When we got a chance for a free TB from BBK (lend them our car to test a new TB design and we can keep the TB), we jumped at that. The car and associated dyno run (which looked a lot like a stock market graph instead of a smooth power band) can be seen here:
Little did I know the car had an atrociously dirty MAF and a CAT that was for a 2" exhaust installed. This car had been necked down from 3" to 2", the cat installed and then necked back up to 3". I found these out and corrected both (cleaned the MAF and installed 3" Magnaflow CAT) when I tried to smog the car.
Then my son goes and cuts off the muffler to make it loud and proud.
Come to pass that my son blows a radiator hose and overheats the engine, blowing the head gasket. So we get the engine rebuilt. .030 over on the pistons, new timing chain, new gaskets, refurbished heads (he cracked both on the overheat), refurbished cam, new bearings, etc.
My problem is that after the rebuild, the car seems to drop 100-200 rpms and bog under light accel, but not all the time. It also smells a little like gas and get very bad gas mileage (10 or under with 65% freeway driving). From my time working on crotch rockets, it feels like an air/fuel problem.
Since everything on the car was 20 years old, I have slowly replaced almost every sensor on it knowing it is a censor driven car. Items replaced:
Throttle body: BBK
MAP Sensor: BWD
Evap Solenoid: BWD
Water Temp (Both): BWD
AIT Sensor: BWD
MAF: Delphi
02 sensors: AC Delco
Ignition Coil: BWD
Optispark: AC Delco/Delphi
Fuel Filter
EGR Valve: BWD
New Plugs (AC Delco) and Wires (7mm aftermarket)
Water Pump & Radiator
Items still left:
Throttle Position Sensor
Idle Air Sensor (bottom of TB)
Oil Pressure sensor
Fuel Pressure regulator
ICM
Knock Sensor
There is also a sensor down by the timing chain cover that has not been replaced.
ICM (but has been spaced off the block to prevent heat soak)
We have tested the fuel rail and it maintains 42psi with the key on and 36-7psi started and at idle, like the Chilton manuals says it should, so we don't suspect fuel pressure issues.
What we did find was most likely 20 year old fuel injectors with crappy/hard O-rings. We rigged up a test setup and found one injector that literally flowed fluid under pressure without any power and had some kind of hard precipitate in it that turned to a white ashy substance when it dried out. It also shot a **** stream when actuated. Replaced with BDW unit and replaced the O-rings on the remaining injectors. Test run last night was superior with a definite reduction is gas smell. This morning, drive to school was fine, but a quick run to the store a little later (restart after warm-up) and the same issues.
None of the other injectors spray like the new one. We tested it before install to see what a new/correct pattern should look like. They all spray varying levels of a stream surrounded by a cone of mist, whereas the new unit sprays a dense cone of mist. Also, my son said that two other injectors very slow-dripped under pressure but did not allow a flow of fluid through like the bad one. My understanding is that injectors should not allow any fuel through unless under demand.
So I am stuck, but maybe not: I am thinking of just replacing all injectors with 24# Accels. Philosophy being that everything is 20 years old and replacing a few could still lead to problems later. Is that the right approach and is it heading in the right direction? Am I missing something? The old pistons and valves had tons of crud and crap on them. Is it possible for an engine management system to slowly tune down as the engine system degrades over many years? Does it run bad at low throttle because my loud and proud son cut off the muffler and there is no back pressure? Did he jam the CAT with crap when he blew the head gasket?
A little bit of advice here could save me 4-5 Benjamins, and I am not too proud to say that I have reached the end of my knowledge base/skill set.
Thanks in Advance,
Sean
1994 Trans Am GT, 5.7L, BBK TB, no muffler, everything else stock.
10-25-2014, 05:17 PM
#2
Just my 2c, I had a 93 LT1 that ran great until it warmed up. It stumbled all over itself and backfired EXTREMELY bad. I replaced the entire ignition save for the ICM and it didn't fix anything.
Replaced the ICM and poof, problem fixed. You have it spaced to prevent heatsoak, but the damage may already be done. I would suggest you at least take it to a parts store and have them test it.
Replaced the ICM and poof, problem fixed. You have it spaced to prevent heatsoak, but the damage may already be done. I would suggest you at least take it to a parts store and have them test it.