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Automotive/irregular pop sounds in exhaust

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Expert: - 12/12/2007


Question
QUESTION: Hi,

I drive a 1989 Nissan 180SX with a Motec aftermarket ECU. Recently, due to the bout of rainy weather, I'm starting to hear irregular pop sounds coming out of my exhaust.

A mechanic said that my "cold start" are in the ecu needs more fuel cos I'm running lean while a friend suggested that my plugs could be wet due to moisture and the fact that I have a huge vent in the hood directly above the engine.

What are your thoughts?

thanks a bunch in advance,
Sean

ANSWER: I would agree that exhaust popping sounds are the hallmark of an air/fuel ratio that is on the lean side, and that would suggest that the mechanic's advice regarding ECU set-up would be the best to follow.  (My own hands-on familiarity is with carburetor tuning, and I would look to too lean a setting of the idle bleeds.  Note that, being an electronics engineer, I tend to view my automotive pursuits as an electro-mechanical "busman's holiday!)

Wet plugs?  Definitely not in the combustion chambers!  The only effect of moisture on an ignition system would be a moderate to severe loss of spark energy, and thus of ability to start, ability to load, and general driveability.  Once the engine has been running (at idle) for a while however, that sort of problem would pass as the high-voltage wiring and connectors dry out.  This is not your present problem however.

Regards ... EGK

---------- FOLLOW-UP ----------

QUESTION: Thanks for the reply Ernest :)

Actually what my friend meant was that the ignition coil area might be wet and due to the dampness of the air (rainy season..) there is some sparking and voltage leak. Is that possible?

I'm still a little doubtful of the lean mixture theory cos before the rainy season, I had no such problems. In fact, I was told that my exhaust reeked of fuel cos its too rich. However, despite that, I increased the amount of fuel in the "cold start" and "engine temp compensation" areas yet the problem persists.

puzzled :(

Answer
"Sparking and voltage leak" results in a loss of spark energy, and its attendant problems.  As to relative humidity (damp air), only a very old system, one with spongy and porous insulation would be affected by that type of dampness (I am inclined to add that the same would be true for most English (Lucas) electrical systems of the sixties ... Austins and such were TERRIBLE for ignition reliability in wet weather ... how such designs could originate from a country with ample experience with rain, I cannot fathom!)

Exhaust popping on one hand, and anecdotal accounts of fuel smells in exhaust on the other suggests variability, and variability is a common indicator that the system is not fully in tune, ie: settings are not quite to specification.  My best advice would be the time-honoured approach of using a tachometer, timing light, vacuum gauge and, if you can get one, an exhaust analyser to set the engine up to meet its designed specifications.

The proposed above is a thorough and systematic approach, and is more productive than chasing after symptoms that appear to change.  In my experience, this always works.

Regards ... EGK

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Ernest (Ernie) Kenward

Expertise

The challenges I most enjoy are thoughtful technical questions of a trouble-shooting nature in both electrical, power electronic and mechanical systems, mainly automotive but also machine control and small-machine PLC applications. Please note, however, that I am NOT a walking shop manual! I DO, however, make it a point to have those manuals and other service literature for those vehicles I DO own, and highly recommend that anyone serious about maintenance or modification of their vehicles do the same; MOST of your answers WILL be found there. For that matter, I do NOT go out of my way to acquire shop manuals for any vehicle I do NOT own! That being the case, any general query to me along the lines of "What is the meaning of this code read from the ECU of my 2015 XYZ?" or "Where is the fuse for the windshield washer pump found?" (try your car's electrical distribution panel for a start!) will not go far. What I do offer is a pretty good collection of literature, insights and hands-on experience with 1950s to 1980's Ford products (plus a developing database of information and practice with the Mercedes diesel cars), along with an engineering perspective and the ability to design and implement custom control, electrical and mechanical subsystems for vehicles. For that reason, I am happy to make my thoughts and efforts available to those who are of like mind and/or are seriously making a point of learning about their vehicles. Use the Opportunity to Learn!

Experience

A key skill in my work and hobby pursuits both is STRATEGIC TROUBLESHOOTING. I am a senior instructor in Electrical Engineering Technology at a leading Canadian polytechnic, my areas being Electrical Power and Industrial Control, electrical and electronics design and manufacturing, and AutoCAD and related CAD/CAE software - plus equipment problem-solving and new equipment design and prototyping. Hobby-wise, I have 30-plus years of experience in auto restoration, mostly in electrical and mechanical systems. Ongoing projects include a 1959 Edsel Corsair, my 1978 Ford E250 class-B motorhome conversion, and the care and upkeep of my Mercedes 300CD. My vehicles become engineering test beds for electrical and mechanical upgrades as ideas present themselves. This includes the design and production of circuit boards to restore or enhance features for which no OEM replacement parts are obtainable, or where better specifications or reliability can be had via newer concepts. Regarding the E250 RV conversion, I designed and continue to revise a custom power distribution system, managed by a Programmable Controller (PLC); this has made most revisions as easy as uploading new firmware as I develop it. The "mini" PLC is a powerful device for custom automotive control systems. One good example (there are many) would be the Moeller "Easy Relay"; these offer a wealth of control, monitoring and variable-and-status display options for such projects. A good example project which has worked well is that one for my RV noted above, which has been on the job - revised in firmware only - for a decade now. It is a load management and charging control system to avoid the sulfation-induced early failure that often befalls deep-cycle batteries used in RV power applications. The battery installed in 2003 lasted long enough to more tnan pay for the PLC that contributed to its longer life ... and the PLC will be there for the next battery as well!

Organizations
IEEE - senior member ... past WCC Student Activities; SME - senior member ... past chair, greater Vancouver chapter chair 318; Edsel Owners' Club - have served in various capacities on chapter executive during seventies; have been Power and Driveline resource on the Edsel Owners' Club "E-team" for more than a decade.

Education/Credentials
Graduate of UBC

Awards and Honors
Certificates of appreciation from IEEE and SME for work in student and chapter activities

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