Car: 1988 Z51, ported TPI intake, runners, plenum. American Racing Headers (long tube 1 7/8), 3" xpipe, 3" Corsa's. Custom chip from MrWiley with Moates adapter, EGR Removed, AIR Emissions removed, Cooling upgrade (160-180) double fans.

Issue causing me to check timing: Under light throttle, the exhaust will pop a little out the tailpipes. If you are driving a slightly accelerate, then let off the gas, you get a few mild pops out the tail pipes.

Timing the vehicle: Let the car run to operating temp, shut off the engine and unplugged the EST wire (located by the brake booster and wiper motor). Started engine, attached timing light and noticed ignition timing was at 0. Bumped timing up to 6 degrees, seemed to idle ok. Cleared code, started engine and drove.

In driving the vehicle with the clutch depressed after driving would almost stall and the popping I am hearing in the exhaust seemed slightly worse.

I re-checked timing and it's 6 degrees, but then I checked timing with the EST wire connected and the timing was way advanced off the scale. I then set the ignition timing to 8 degrees with the wire still plugged in and the idle was much better with just a slight pop in the exhaust under medium throttle. Re-checked timing with the wire unplugged and it was back at 0.

Prior to do doing all this I checked the TPS and AIT and performed the test as described in this forum and moved the TPS to .60 instead of .49 as it was prior.

At idle you can hear a very faint pop in the exhaust, figured it was timing related or due to the fact that the exhaust is 3" and prob over kill for what the engine needs to breath and smashing cooler air and exhaust air in the tubes.

Could use some help here, I am about done with this TPI set-up.

 

I have been having a lot of these problems too, and some of them I just live to deal with, but the big ones I have been weeding out.

But what I have learned from all this, is ignition timing will cause backfire out either the intake, or exhaust.

If it backfires out the intake, and is related to timing, timing is too advanced, if it backfires out the exhaust and is related to timing, timing is too retarded. Set the timing with the EST wire unplugged, you will get Electronic Spark Timing advance at idle, which is why you unplug the wire to set it.

Another thing that will cause backfires, leaks. Vacuum leaks and exhaust leaks. Vacuum leaks cause a backfire out the intake, because if one cylinder runs too lean, it will take a longer amount of time to burn, and will go back through the intake valve during valve overlap and finish igniting in the intake.

If you have an exhaust leak, as the fresh air enters the exhaust it will help burn the unburned exhaust gases causing a pop or backfire.

Running too lean will have the same effect as a vacuum leak in terms of backfiring.
Running too rich will cause the air/fuel mixture to take longer to burn and cause the gas to finish igniting inside the exhaust. Now you may not have this problem if you are a little rich. But with headers, if you run a little rich, what's left of the unburned mixture may hit the hot header and ignite quickly, which is why you may hear an exhaust backfire near the engine.

Also improperly adjusted valves will cause this, if an intake valve is too tight, it will not fully close when the spark plug wires and an intake backfire will occur. If an exhaust valve is too tight it will not fully close when the spark plug fires thus leading to an exhaust backfire.

A bad spark plug or spark plug wire can cause a backfire either out the intake or exhaust. Same rules apply. If you have a plug or wire arcing with half the voltage going to the electrode on the spark plug, it will be a colder spark, and take longer to burn the air/fuel mixture. This can result in an intake backfire due to over lap, or an exhaust backfire.

And last but not least, crossed spark plug wires. If you cross the spark plug wires, those two cylinders will fire out of time. If you get intake backfires then whatever crossed wires are firing a cylinder too soon, if you get an exhaust backfire then whatever crossed wires are firing a cylinder too late.

Good luck, hope this helps! It's a real pain in the ***!

 

You were setting base timing correctly. With EST connected, timing should advance as you increase RPM, and will also do that with EST disconnected if you advance throttle, at least my 91 does. I believe there is some advance built into the ICM.

Even at 0 you should not get popping out the exhaust, but will notice hard start and less response.

I would look elsewhere for the solution. FSM mentions low coil output, plugs, maybe cracked, cross-firing, etc, distrib cap damaged, wires crossed. Also leaking valve, valve timing.

I would check the easy stuff, ignition related.

 

I'd also get an AFR gauge and Wideband oxygen sensor because it sounds like you may not be getting enough Acceleration Enrichment, causing a lean spike.

 

You are correct in checking the timing with the est wire off. It should be set at 6-8 degrees. When you plug the wire back in, the timing will advance to 18 or 20 degrees which will be off the timing tab. You will need a degreed balancer or timing tape to read it.

 

set timing as you did with est wire disconected at 6 deg. You have other problems then timing causing your poping

 

I like to set the TPS to .65 Vdc at base idle of 450rpm or ~.75 Vdc at street idle of 700rpm.

I agree on checking for a vacuum leak in the manifold.

Do you have installed an AFPR? Has the chip been programmed to increase fuel flow at WOT?

 

I did run through the entire car looking for vacuum leaks and replaced a few lines, the car did not pop prior to the exhaust change, the wires are correct and car is not missing at all which would indicate improper firing order.

On the passenger side, I can hear a light hissing noise under load coming from the header t the number 2 cyl. But there is not carbon deposit on the head and I can not feel any exhaust leakage and all header bolts are tight. I do not think I have a vacuum leak as the engine is running good and the plugs do not display a lean condition...

I am not sure why when the car was timed at 6 degrees, it would barley idle only after driving it. Seems the more advanced from 0 to 6, the worse the popping and idle would be.

Currently the timing is back to 0, the way it was before -

So I have done the following:

1) Checked for vac leaks.
2) Adjusted TPS/AIC.
3) Checked/adjusted ignition timing.

Should I be looking at fuel next? Maybe a bad injector?

 

The chip was programmed to work with the current mods by MrWiley, if I remove the chip and run the OEM chip, I get the same issue but then I get poor throttle response.

I am not sure what you mean by "AFPR"?

 

Adjustable Fuel Pressure Regulator.
Turning up the Fuel Pressure will make it richer, and turning it down will make it leaner.

 

I just want to clarify: Spark map used is based on the 89 ARAP with 30 degrees at WOT with the PE spark map set to zero. With initial set to zero it is only going to see 30 degrees total, and really should be set to 6.
I'm not sure what's causing said popping (would rather not speculate), but wouldn't think it's timing related considering 1000's of others including all stock 89's running this BCC do not experience this issue.
Shawn, please keep initial at 6 degrees and search for other causes. Your stock 88 memcal could be tested, and is slightly less aggressive than the ARAP, but would prove it is not timing related. It will set a SES light due to items deleted, but could be done as a test.

 

I'll move it back to 6 degrees, wondering if it's a fuel issue at this point. I am gonna triple check for vacuum leaks.

Anything else i should be looking for?

 

using a scanner observe blm counts. Close to 128 is perfect, below is rich, and above is lean. Taking readings at various points of speed and load will give an idea of fuel conditions. An AFPR is a great tool for tuning fuel as already mentioned.

 

Everything I Mentioned

 

Actually, turning up the regulator (unscrewing the adjustment screw) will make it leaner. Turning it down will make it richer.

 

I am triple checking it all tonight, thank you.

 

It's time to get a scanner.

 

Make sure your harmonic balancer outer ring hasn't shifted. It will affect the timing, obviously.

Here's a pic of what it should look like (assuming you have a stock one, of course):

 

I did not even think to check the balancer, mine as well check that out. I found a few things tonight. I pulled the cap because I found the number 1 post was bent and loose. Wondering if that could effect the spark at #1 if the lead is loose inside the cap?

Also found a bare wire under the cap, the wires coming from inside the cap - one was stripped and may have been grounding put on the HEI housing.

Re-checked all plugs, pulled all spark plugs and checked gap and serviceability.

I will set the timing tomorrow, did not want to wake everyone up

 

Was playing around on Youtube and in this video: at :33 seconds, that's exactly what the popping is and sounds like in my car, maybe just a tab less in my car.