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beatnavy's Avatar
 
Join Date: Sep 2013
Location: Northern VA
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Exhaust Popping

My 72 1.7L stock D-Jet has been running well since I brought it out of a long slumber over the last 6 months, but the last couple of weeks it's been popping (e.g., small backfires) after I let up on the gas. I want to start troubleshooting this some over the next few days. I'm looking for help prioritizing my efforts. From other posts I know it may be a lean mixture or exhaust leak. Not sure what to do about the lean mixture other than check fuel pressure as my ECU is the kind with no adjustment knob. Here's some other info:

1. I just checked the timing and am pretty sure it's right on the mark.

2. Last time I checked dwell it was probably around 52 - I know the range is 45 to 50, but that's the closest I could get it last time I was adjusting it.

3. Car idles pretty well, and I'm not aware of any significant vacuum leaks.

4. I checked fuel pressure a couple of months ago - I think it was about 30 lbs.

5. Fuel system is almost completely new or reconditioned - injectors were serviced and I replaced condenser, points, wires, plugs, and coil on ignition system.

Not sure how best to check exhaust for leaks. After some inspection I don't see anything obvious. Anything else I should put on my checklist? Trigger points or other parts of ignition?

Thanks for the help in advance.
Old 05-03-2014, 04:46 PM
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As a follow up, I did the following:

1. Checked fuel pressure. Gauge bounces around a bit, so it's hard to tell exactly, but it looked like it was hovering around 29 psi. I upped it very slightly.

2. Checked vacuum hoses, cleaned trigger points connectors, and made sure FI connections were solid.

3. Checked spark plugs and spark plug wires.

Popping seems like it may be a little better when I downshift and coast after high RPM, but it was hard to tell given how windy it was outside.

I'll keep watching (and listening).
Old 05-04-2014, 01:53 PM
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It does not take much to create a vacuum leak and if I remember correctly that is a big cause for exhaust popping. I would suggest doing the carb cleaner spray trick around ALL the intake hoses, joints and connection. Several places that can leak after a prolonged sitting are the intake runner to head joint as the gasket dries out and any of the cloth covered hoses as they can hide a crack very easily. You will have to put the car on stands to run it and listen with your Harbor Freight stethoscope around the exhaust gaskets as they can loosen up also.
Old 05-04-2014, 02:48 PM
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Air being pulled into the exhaust system is probably the most common of popping through the exhaust. Unburned fuel entering the exhaust system would be the next most common cause. If it's popping through the intake, a vacuum leak, causing the engine to run lean, would be the most likely issue.

The Cap'n
Old 05-05-2014, 12:27 PM
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Thank you, gentlemen. This evening I tried the brake cleaner trick, but I did not notice any significant change in idle as I sprayed it near the usual suspects (intake runners, AAR connections, throttle body). I could be doing it incorrectly, but vacuum leak may not be my problem. When I have an opportunity, I'll try to check exhaust joints (although the forecast is for rain for the next few days -- not taking my baby out in the rain!).
Old 05-05-2014, 03:36 PM
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