Is it normal for exhaust to suck air?

As the title says, is it normal for exhaust to suck air?

Some background iv got a stock 350 vortec crate engine from jegs with a tbi and a 7427 computer controlling it along with true dual 2.5 inch exhaust pipes with an X-pipe midway back. Iv been facing some rough idle issues with the truck the whole time iv tried tuning it and its very frustrating.

At idle with a napkin over one of the exhaust tips it gets sucked in pretty quick and spat out over and over. Iv read that this is exhaust reversion but is this normal?

When doing the engine swap i broke an intake bolt and had to take the driver side head off to get it out then lined everything back up and torqued it down. My buddy said it should be fine since it had never been ran, so i never checked anything with a feeler gauge, just put it all back together.

I have not put many miles on the engine, less then a 1000 so i know it hasn't been broken in yet.

Im still new to tuning but i have the computer set up for open loop and have the tables set accordingly to the AFR i want to run at idle. When it runs rough there is no fluctuation in the wideband at all during idle. Im no expert but i want to say timing is causing the shaking of the truck, but im also not sure if its something mechanical instead from putting that head back on. When i first start it and its warming up however the idle is fairly smooth until i drive it and stop at a light then a switch almost flips and it starts idling bad and shaking the truck.

Hopefully you can make some sense of what im saying. Im just to the point im having more failures then success getting this truck to run good and be reliable and its getting overwhelming and costing money.

In order for a piece of paper to be "blown away" from the exhaust all the time, there has to be positive pressure at the exhaust tip. When you enlarge the pipes and split the exhaust between two pipes, you greatly reduce the pressure at the tip. Between exhaust pulses this pressure could approach zero with a stock, mild cam. This would make the paper bounce away from the pipe then return to it's rest position. But If the piece of paper is actually being "sucked in" I would suspect the valves need adjustment.

You could also have a misfire causing large variations in pressure. An electrical issue could explain the sudden change. Electrical problems are sometimes likely to show up when an engine is warm.

What tools do you have available for testing?

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Originally Posted by 1project2many

In order for a piece of paper to be "blown away" from the exhaust all the time, there has to be positive pressure at the exhaust tip. When you enlarge the pipes and split the exhaust between two pipes, you greatly reduce the pressure at the tip. Between exhaust pulses this pressure could approach zero with a stock, mild cam. This would make the paper bounce away from the pipe then return to it's rest position. But If the piece of paper is actually being "sucked in" I would suspect the valves need adjustment.

You could also have a misfire causing large variations in pressure. An electrical issue could explain the sudden change. Electrical problems are sometimes likely to show up when an engine is warm.

What tools do you have available for testing?

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I have electrical testing tools, no feeler gauges but i can pick some up. Also the napkin gets sucked into the exhaust tip, I actually thought I lost it until it shot back out. The truck has always had gremlins in the electrical so that is possible.

I had a thought when writing this, months ago the truck started acting up in closed loop when I let off the throttle. The wideband would swing way lean then way rich until I gave it throttle again then it would recover and run fine. Is it possible the side the NB o2 is on is sucking exhaust back and causing issues with the fueling? When ran in openloop the truck runs great aside from the idle.

I can upload a bin if needed along with a datalog too. I'm still not to sure how to understand datalogs as they are still Greek to me. I know what stuff kinda means but not what causes what or what exactly needs to be changed to fix a problem.

Also its worth noting the basic TBI stuff has been checked such as timing at 0 and fuel pressure is steady and strong. I actually have a TPI pump and an aftermarket FPR. pressures have been accommodated in the tune.

Where should I start testing? Sensors or pull the valve covers off and go from there?

i believe if it was me the first place i would start is checking lifter adjustment and a compression test (rule out mechanical, burned valve ect)...........

Your napkin test sounds normal to me. I would start by running the valves. Get engine eto operating temp, then one by one, loosen the rocker nut until it clacks, then tighten til its quiet, then tighten another 1/2 to 1 turn. As it is right now, you might be having valve lash issues since you had to take the head off the new engine.

When your engine starts running bad like when you flip a switch, I'd bet that's when the ECM is entering Closed loop.

WHat are you using for an intake and what are you using for injectors?

peace
Hog

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Originally Posted by Hog

Your napkin test sounds normal to me. I would start by running the valves. Get engine eto operating temp, then one by one, loosen the rocker nut until it clacks, then tighten til its quiet, then tighten another 1/2 to 1 turn. As it is right now, you might be having valve lash issues since you had to take the head off the new engine.

When your engine starts running bad like when you flip a switch, I'd bet that's when the ECM is entering Closed loop.

WHat are you using for an intake and what are you using for injectors?

peace
Hog

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The intake is an edlebrock along with a 350 TBI and I believe 61lb injectors. I have them running at 20psi and have the bin adjusted to them. The spray pattern looks good too.

I have the bin set for open loop only and the o2 sensor is unplugged. I do have a wideband hooked up so I can monitor the fuel mixture.

I will try to check the valves as soon as I can then.

Thank you.