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3.4 swap crank no fire, popping and backfiring.

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Old 12-03-2018, 01:44 AM
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Angry 3.4 swap crank no fire, popping and backfiring.

Did a much needed over haul and decided to pull the engine and trans to replace a cracked crossover with one from TOS and the clutch while I was at it.
Put everything together loosely and did a test start today that was unsuccessful.

Crank and cam marks all line up and I have been through the spark plug wires at least ten times for the correct firing order
cam sensor is plugged in
crank sensor is plugged in
maf plugged in
tps and iac both plugged in

Edited* I did an ECT mod so i can change the coolant temperature the ecu see's and just realized that's one thing I didn't plug back in.
Will it run without a input from the engine coolant temp sensor? Would try but it's 2 am and no muffler hooked up lol.



Backfires and pops (exhaust and intake) like its the wrong firing order but isn't.
any ideas?

Last edited by david braun; 12-03-2018 at 02:19 AM. Reason: Realized something that I forgot to add.
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Old 12-04-2018, 11:51 AM
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An unplugged coolant sensor will make it run pig rich, usually enough to keep it from starting. If its unplugged or you did your mod wrong then that definitely could be your problem. Did you use a potentiometer for your mod? make sure it isn't set for cold.
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Old 12-04-2018, 11:04 PM
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Thanks for the reply, It's getting fuel for sure, (can smell it) Just popping out the intake and exhaust. Everything's lined up just forgot to plug that damn sensor in and have been at work.
Going to try tomorrow and see if that was the issue.
Will check back.
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Old 12-05-2018, 04:53 PM
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Well this is the verdict.. compression check shows 2 cylinders are well below spec just not sure would cause it.
Burnt valve is my best guess at this point(drove it 15 min at 40 mph with a broken exhaust crossover
Would it still run on 4 cylinders? Timing is dead on and all the sensors the ecu needs to run where plugged in.
Has fuel and spark.
Video if it cranking






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Old 12-05-2018, 08:13 PM
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Timing looks fine. Pull the circuit opening relay (fuel pump) and try again. It sounds to me like its not starting because its too rich caused by the ect sensor unplugged. Even with two low cylinders it should still try to start when fuel and spark are right.

As for your compression problem, its not likely it burned a valve from that. A cracked crossover won't really let in enough cool air to cause harm. You need a short and large opening for that to happen, like a an open manifold. Its possible the cylinders were already a little low and then the walls got washed from flooding. Does your oil smell like gas?

Consider retesting. Pull all the plugs. Varying compression numbers between cylinders changes the speed of the motor as it goes into the next compression stroke. This causes compression numbers to vary more than they actually are. Pull all the plugs and charge the battery so you get the most accurate test. One thing I like to do is test multiple times. I like to do 3 test per cylinder. Many times have I gotten a dud reading because the guage bleeds down when in a certain position.

With that all being said, it does sound like it has lower compression on a cylinder but doesn't sound like its as bad as your numbers suggest. It doesn't explain why it doesn't even try to start. I'd remove your ect sensor mod. Its possible its wired wrong. Also make sure there isn't any pinched wires or anything. I've seen harnesses get pinched in the bell housing many times.
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Old 12-06-2018, 03:49 AM
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After Freaking out thinking i might need a new engine or rebuild I did some more research on what would cause a no start with fuel, timing and spark.... Air (Duh)
Pulled the IAC valve apart and the valve was frozen closed..Would explain the no start and the low compression. During the compression tests and the times i have tried to start it i Never once cracked the TB.
So I'm hoping that's the issue but do to work i wont know till Sunday
I will also look into the things you mentioned
thanks.

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Old 12-08-2018, 05:56 PM
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Fixed!
Well it runs now and I'm calling that a win. Now to fix the miss, and see if the clutch works..
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