Tbi engine backfire

[72chevy2door]

Hi guys I'm new to this forum and would greatly appreciate y'all's help. I own a 1988 gmc 1500 suburban. It has a 350 small block with tbi injection and a 700r4 transmission. I just did a fresh rebuild on the motor built it completly back to stock other than the camshaft, timing chain, and long tube headers. The camshaft is a speed pro cam part no. CS-1014R. The timing chain is a cloyes heavy duty double roller. The headers are hooker headers competition darkside part no. 2452-3HKR with 1 5/8 primaries and 2 1/2 collectors. The problem I'm having is at first start up it runs fine then after about 30seconds it starts to backfire. The backfiring is only conning from the passenger side exhaust. I'm sure I'm not the only person with this set up so if someone can help me with this I would greatly appreciate it. Thanks

 

[Jims86]

Hi guys I'm new to this forum and would greatly appreciate y'all's help. I own a 1988 gmc 1500 suburban. It has a 350 small block with tbi injection and a 700r4 transmission. I just did a fresh rebuild on the motor built it completly back to stock other than the camshaft, timing chain, and long tube headers. The camshaft is a speed pro cam part no. CS-1014R. The timing chain is a cloyes heavy duty double roller. The headers are hooker headers competition darkside part no. 2452-3HKR with 1 5/8 primaries and 2 1/2 collectors. The problem I'm having is at first start up it runs fine then after about 30seconds it starts to backfire. The backfiring is only conning from the passenger side exhaust. I'm sure I'm not the only person with this set up so if someone can help me with this I would greatly appreciate it. Thanks

First thing, If you have not done it already, you need to install a heated o2 sensor in the drivers side collector. Not saying this is the cause of the backfire, but is necessary to do any further diagnosis.
Also, TBI is very sensitive to cam change, especially a cam that will affect vacuum.
do you have the specs?

 

[72chevy2door]

Thank you for the response. Yes I do have a heated oxygen sensor on the driver side collector. The sensor is for a 1994 chevy suburban 1500 w/350tbi. It has three wires on it; two white one black. One white is ground the other is 12v switched source and the black goes to factory o2 sensor wire. I was told that's how to wire it so I hope it's right. I don't know how to test if it is working or not. By the way there is no check engine lights. The cam specs are in the following image

 

[Jims86]

Thank you for the response. Yes I do have a heated oxygen sensor on the driver side collector. The sensor is for a 1994 chevy suburban 1500 w/350tbi. It has three wires on it; two white one black. One white is ground the other is 12v switched source and the black goes to factory o2 sensor wire. I was told that's how to wire it so I hope it's right. I don't know how to test if it is working or not. By the way there is no check engine lights. The cam specs are in the following image

Ok, thats maybe a step above stock, lobe separation of 112 is good, duration is a bit more than stock.
Sounds good on the o2 sensor.
So now, I would like to see what you get for fuel pressure. 9-13 is a good, factory spec reading. any lower, and there may be a leak in the hose between the fuel pump, and sending unit in the tank, which we have seen a few times. Its one of those hidden problems that drives you nuts.

 

[72chevy2door]

According to northern auto(company I bought engine rebuild kit from) claims this camshaft is computer compatible, summit tech support says the same thing. I chose this cam because it's not supposed to require a chip or anything and it's supposed to be a RV style cam. I have replaced the knock sensor(broken) both oil pressure switches(one leaked from plug other didn't work) and I have replaced the coolant sensor on the intake(was told this May fix it). I haven't been able to start it yet to see if it fixed it. Also I drained all the gas from the tank and put a new fuel filter on the frame rail just in case the gas was bad somehow. The pump seemed to have pushed quite a bit of fuel out but I will see if it is getting the 9-13psi. Should it be constant pressure and how should I hook up the gauge? Use a "t" fitting? How do I test to make sure the o2 sensor is working? Thank you for all the help so far

 

[Jims86]

According to northern auto(company I bought engine rebuild kit from) claims this camshaft is computer compatible, summit tech support says the same thing. I chose this cam because it's not supposed to require a chip or anything and it's supposed to be a RV style cam. I have replaced the knock sensor(broken) both oil pressure switches(one leaked from plug other didn't work) and I have replaced the coolant sensor on the intake(was told this May fix it). I haven't been able to start it yet to see if it fixed it. Also I drained all the gas from the tank and put a new fuel filter on the frame rail just in case the gas was bad somehow. The pump seemed to have pushed quite a bit of fuel out but I will see if it is getting the 9-13psi. Should it be constant pressure and how should I hook up the gauge? Use a "t" fitting? How do I test to make sure the o2 sensor is working? Thank you for all the help so far

Pressure should be constant, between 9-13 psi. there is a fitting that comes with the proper gauge to install in line on the feed line at the TBI unit.
An o2 sensor in bad enough shap to cause a major problem like this would set a code.

 

[farmerchris]

since the backfire is on the passenger side only,did you get the valvetrain adjust down right,maybe little to tight ,causing backfire through the valves take only one or two being tight.

 

[Jims86]

Yes...Check the valve adjustments if its only popping on one side.
Jeff, did you do the rebuild, or a builder..... i missed the pass only backfire part.

 

[Mattchu60]

I may have missed it above, did you install new plug wires and distributer cap as part of your rebuild? Mine was backfiring and I chased a lot of issues before I figured out I had bad plug wires (they looked good on the outside).

 

[72chevy2door]

@jims86, I have not been able to check fuel pressure yet. @farmerchris, I adjusted the valve lash like I have done all other small block chevy and fords by tightening down the rocker until no up or downward movement is left in the pushrod then going another half turn to set lifter preload( that's with lifters primed up with oil and at base of camshaft). @Jims86, I did build the motor completely. The only thing I didn't do was the block and rod machine work(cleaning and block hone- rods had to have big end ground circle again) and the head machine work(cleaning and new factory rated springs). I plastigaged the whole bottom end like I've done all motors with correct clearances. I will go back and check the valve lash though, I've seen rocker studs pull through the head and cause lash to change. @Mattchu60, yes new rotor button, distributor cap, sparkplugs, sparkplug wires. And I put a new ignition in the truck also to see if that could be my problem. I am a carb guy so much easier but I like the reliability of the tbi so I wanted to retain that if possible. Hopefully I will be able to get to work on her today. Any other suggestions for me to check other than valve lash, fuel pressure? And thank you all for the suggestions and help so far

 

[72chevy2door]

Alright guys I been working on my Burban and I got the fuel pressure and readjusted the valves. It doesn't backfire while in park or neutral, when I place it into gear she will backfire. She does idle fairly rough. Me getting impatient and trying to test drive it didn't change anything. I did learn she has absolutely no torque. She does stall out every now and then. I am running 93 octane fuel in it. I'm wondering if the distributor is bad or maybe the coil. The sensors on the bracket next to the throttle body I'm wondering if they could cause it? What do y'all guys think?

 

[Jims86]

Alright guys I been working on my Burban and I got the fuel pressure and readjusted the valves. It doesn't backfire while in park or neutral, when I place it into gear she will backfire. She does idle fairly rough. Me getting impatient and trying to test drive it didn't change anything. I did learn she has absolutely no torque. She does stall out every now and then. I am running 93 octane fuel in it. I'm wondering if the distributor is bad or maybe the coil. The sensors on the bracket next to the throttle body I'm wondering if they could cause it? What do y'all guys think?

Have you confirmed the timing setting? 0* BTDC Coil is plausible for the rough idle, especially if it's aftermarket. Just had a guy recently with problems. Had an MSD coil I think, he slapped the delco coil back on and it cleared up. But that wouldnt cause backfire. Backfire would be lean mixture, a broken or moving pickup coil in the dist, crossed wires(5-7 are common, 4-8 on occasion) do the easy one first, and go through the wires. After that, we will check the voltage of the MAP sensor. you have a DVOM?

 

[72chevy2door]

It has the original coil from 1988 and the distributor is original also. I will double check the wires again, it is very possible for me to of crossed them by accident. I haven't even thought of checking them. Lol. This truck has made me want to burn her down cause all the stress it's caused me. The sensors on the bracket how would I test those?

 

[Jims86]

It has the original coil from 1988 and the distributor is original also. I will double check the wires again, it is very possible for me to of crossed them by accident. I haven't even thought of checking them. Lol. This truck has made me want to burn her down cause all the stress it's caused me. The sensors on the bracket how would I test those?

Well, I am sure we have seen just about everything, or damn close to it. The only thing that really eludes us in the EFI realm, is the sudden rich running that these systems start doing all of a sudden.

Be sure to check the magnetic pickup coil inside the dist as well. Make sure the outer relluctor is not moving. needs to be solid, otherwise, it is like having the distributor loose and moving back and forth ****** the timing up.

 

[72chevy2door]

Hey Jim86 I haven't been able to get to my garage to check the distributor on the truck. Is there any advice you can give me on how to test the sensors on the bracket next to the throttle body?