hey guys,

Here's what I'm looking at, 94 V6 3.4L, 5sp, stock everything.
Car has 127K miles, motor has 25K, rebuilt by Jasper
This car is new to me, so past history is hard to come by.

Here is what I have replaced:
Fuel pump, fuel filter, plugs, wires, pcv valve, air filter, trans mount

Here's what I have tested and passed:
computer - replaced, no effect, reinstalled old computer
EGR valve - cleaned, functions normally, as far as I can tell
coil packs - ohm tested, OK 5.8 across secondary, 0.3 across primary
cleaned throttle body, IAT sensor
all injectors pulse, I have not checked the spray pattern

Seafoam helped a little, mostly smoothed out the idle, and runs quieter now. It also helped with SES light coming on every time we drive it. Now it only comes on when it backfires.

Here's the symptoms:
Runs fine at idle, and when revved in N
exhaust leak behind cat, but shouldn't really cause this issue
under load, misses throughout all gears during acceleration, but smooths out and surges when let off slightly. Most noticeable in 2nd and 4th.
During deceleration, will backfire both through the intake and the exhaust (not at same time), randomly, but mostly between 3K and 2500 RPM.
SES light does come on after the backfiring, but not during the missing, but we are unable to read codes without a Tech I kit (not worth the cash at the moment). Actron scanner won't pick up codes. OBD I connection, paperclip doesn't work. Smells like it's running rich.

Here's what I am thinking, but I don't want to just throw parts at it if I am out of line... MAP sensor, TPS, or O2 sensors.

Any other ideas from you guys would be greatly appreciated. And if someone knows a good bench test, that would help too.

 

Ohms test is almost useless testing a coil. It will find a dead short in the coil but it will not find spark "leakage". The leakage only happens above 10,000 volts or so.

It still could be something else but with a miss under load I would expect the coil to pass a ohms test. As a general rule coils that fail the ohms test do not fire at all and would give you a miss at idle also. You can find a inline spark tester. Harbor freight sells them for like 5 bucks.

Edit, Paper clip was pre OBD1. I think 93 back. You need a code puller for OBD1, or you can just stop by autozone.

 

tried that, no go, they only have OBD II readers, and their OBD I, which I have, says it will read a 94 camaro v6, but it won't. I am trying to avoid a dealership on that, but I may have to just to pinpoint the issue. Thanks though...

 

Alright, for those who may be having the same problem as I was, here's what I did:

Tested TPS sensor, checked out fine, smooth voltage changes throughout entire throttle movement

Tested O2 sensors, right bank was around .700 @ idle, but when revved up, it fluctuated as expected, left bank was around .350 @ idle, but performed just as the right bank did when revved. Also tested with vacuum hoses removed, performed as desired.

Tested MAP sensor, fluctuated as it should, but not within specified ranges, would jump when throttle was moved quickly, but didn't respond well otherwise. Did test with vacuum leaks, same response. Did not have to remove it for off-the-car testing, as it already indicated it was malfunctioning.

Tested coil packs again, with an inline spark tester as suggested by Gorn, 7 bucks @ Autozone, all seem fine, but I have a spare coil pack just in case.

So I replaced the MAP sensor, tested it on the car, and would you imagine, the problems have ceased. The engine is very responsive to the throttle, seems as thought the engine and drivetrain want to jump out of the car cause the body can't keep up!!!

I have had this car for almost 3 weeks now, and for the first time, it was actually fun to drive. Now to tackle the other issues, a few body parts here and there, paint, rear main oil leak, exhaust, suspension, windshield, and interior repair. One ghost problem down, 10 obvious ones to go.

Here are some videos that I found that helped with the sensor testing, as I am a noob at some of the engine management stuff (I miss carbs), these helped alot. http://www.auto-repair-help.com/auto...deos/index.php

 

Too good to be true of course, not long after the MAP sensor was replaced, the car developed the miss again, but this time, I went straight for the coil packs, ended up replacing the wires and the coil pack for 6-3. Hopefully that will be the last of that.

 

Ok, so after a few weeks of testing, changing, etc. Here is what I ended up doing all together.

O2 sensors replaced
MAP sensor replaced
EGR Valve replaced - had 2 small cracks in the mounting plate
ECM replaced
Plugs/Wires replaced
Ignition contol module and coil packs replaced
Air filter replaced
Oil filter replaced with oil change
Oil pan gasket replaced
Transmission fluid changed
Remounted the exhaust w/new gaskets
PCV valve replaced
PCV Valve harness and all vacuum lines replaced
Transmission mount replaced
and last but not least...
Fuel pressure regulator had a hole in it where the diaphram joins the center of the regulator, allowing some raw gas into the vacuum lines, replaced it, and no more missing, fuel smell, etc. The backfires only happen @ the small leak behind the cat now, throttle is responsive, and I don't feel quite as stupid as I did earlier this week. It kept up with a brand new Mustang V6 off the line this afternoon, and if I wouldnt' have missed a shift, I probably could have taken him.

Hope this helps someone else, I am just glad it's over...

 

Good job bro isn't diagnosing fun its always the last thing you change and usually the cheapest haha

 

email sent to thread initiator -- would be really helpful to compare notes a bit. We would really appreciate it.


Thanks,
Cold_Surfer

 

Hey there cold_surfer,

OK, let me make sure I have it straight....

93 3.4L Auto, 143K, original block, stock except for K&N air filter
you have replaced
both O2 sensors
plugs wires
IAC and TPS
Serp belt/tensioner
Oil Pres sensor
Sensor behind coil packs, this should be a coolant temp sensor AC-Delco PN:213-77 or #25037332
performed an oil change w/filter
replaced the throttle body

First question, map sensor, simple to replace, and dirt cheap compared to some of the other parts
that I have found. it's on the back left side of the plenum, the vacuum hose is a pain to get back on
but other than that, you are looking at 2 bolts, 1 hose, and 1 elec connection. If I remember right, the bolts are metric, just FYI. Here is a link to a video that might help you test it before you replace it.

http://www.youtube.com/watch?v=cFNiq0aNlo4&feature=related

Fuel pressure regulator, there are a couple of tests, but the most obvious one is this, with the car running,
take the vacuum hose off of the regulator, there should NOT be any fuel coming out of the regulator. That was my tell tale sign, mine was appeared as though it was acting like an overflow, not squirting, but flowing steadily. the second of course, is to get your hands on a pressure tester, not going to be cheap, and hook it up to the schrader valve to check consistant pressure. Fortunately for me, my fuel pump was installed just 2 weeks prior to me buying the car, so I knew that was good. To remove it, you will need some Tamper resistant Torx wrenches. I found some at autozone for fairly cheap. Take it apart, clean up the surfaces, replace the regulator (which looks a lot like a thermostat), reinstall and done. The new regulator costs around 40 bucks new. I did not have to pull the intake, but it was a little tight. I did disconnect a few of the injectors, just to give more room for my hands.

With my car, 94 3.4L rebuilt Jasper motor w/25K on 147K body, when I would stomp on it to accelerate, it would fall flat on it's face, still accelerating, but not nearly what it should be, and had a constant miss throughout all gears and rpms. Check your exhaust and see if it's running rich. If the pressure regulator or the MAP sensor are bad, the PCM overcompensates, since there is raw fuel in the intake. I fouled a few plugs until I found the regulator was bad as well as the MAP sensor.

also, check your vacuum lines, I had to replace mine since the "T" next to the FPR was cracked and the previous owner taped it up. I found the whole harness online here: http://www.gmpartsdirect.com part # 10217828
You can't buy just the "T" since it's an odd shape. Local shops wanted 35 bucks and would have ordered it from GM themselves.

I have the GM Service manuals for the 94, which shouldn't be any different than your 93. Just out of curiosity, what was the date of manufacture on the door sticker? what were the codes the mechanic gave you? I can look them up and give you the flow chart if you need.

Hope this helps, keep me posted.

 

ok so I also have a 94 Camaro with a 3.4 by 5speed that was doing the same thing I replaced the o2 sensors and it helped a little bit but I also found that the vacuum hose on top of intake had a brake in one of the lines going to the map sensor so check that out and I bet that's what it's going to be