• Registered: 2011-10-14
  • Posts: 4,167

Topic: Parts swapping verses diagnostics

I run into this quandry every race and ever week or so on a car forum I belong to.  With our (usually) less than modern cars there is a good chance we have a crude diagnostics systems for ignition and fuel injection.  Odds are they CAN point you in the right direction but it often cost significant time and unless you know THAT car well enough to know what THAT code actually means can lead you down a rabbit hole.

In my case, I can tell you from the ODBI flash codes on an LH2.4/EZK system on a Volvo what is really wrong when it says "mixture too lean or too rich" in three guesses.  On a GM ODB1.5 Buick 3800...It takes 10 minutes just to get the stupid computer to sync and pull codes and it does not do stored codes.

So, if you know the car a little and know suck-squeeze-bang-blow is how an engine works, how do yo handle problems at the the track...and conversely when wrenching in the comfort of your garage?

I will give two examples:

We are missing at high speed, when hot with the 3800 Series I in race conditions.  Wires and plugs swapped to no avail.  Engineer teammate plugs in the computer while I have the rest of the team grabbing coil packs from the spare motor.  In the end it APPEARS to have been a bad coil pack but we were also overheating without knowing it so we were knocking so the timing was being retarded.  I threw parts at it and got it back on the track but did not fix it...maybe.

Volvo 1990's 240 will not idle until warmed up.  Hunts up and down and stalls if put into gear until the idle stabilizes after about 5 minutes of idling.  It is taking longer and longer to stabilize idle and now occasionally idles erratically after warm.  Owner throws a NEW IAC (why not just clean it), O2 sensor and cleans the throttle body.  Two questions got me to a diagnostic track "Does the TPS click on throttle open", "Did you adjust or disturb the TPS when you cleaned the throttle body".  Since he did not understand either question, I knew he was a parts swapper.  A five minute diagnostic procedure had him idling perfectly after he loosened two bolts and twisted a little left.

So...at the track, I throw parts at it until it works until the race day ends.  Online and in my garage, I diagnose, test and often repair.  Where do you fall in the spectrum.

  • From: St Louis
  • Registered: 2011-10-20
  • Posts: 2,611

Re: Parts swapping verses diagnostics

Gauges, plenty of gauges!
Basic O2 at least or better yet wideband O2, this will at least confirm or eliminate fuel mixture as a cause. Also fuel pressure gauge is nice if you are missing on track and guessing fuel starve vs electric + heat.
Once you have 100% eliminated fuel delivery then start on electrics.

I say this based on going the random swap out electric parts only to find my misfire was due to not enough fuel.

Apocalyptic Racing - Occupy Pit Lane racing
Racing the "Toylet" Toyota Celica powered by Chevrolet Ecotec.
24x Loser with the Celica. 16x loser in other fine machines
Overall winner Gingerman 2019

Brett85p's Website

3 Reply by OnkelUdo (edited by OnkelUdo 2015-12-29 06:52 PM)

  • Registered: 2011-10-14
  • Posts: 4,167

Re: Parts swapping verses diagnostics

Brett85p wrote:

Gauges, plenty of gauges!
Basic O2 at least or better yet wideband O2, this will at least confirm or eliminate fuel mixture as a cause. Also fuel pressure gauge is nice if you are missing on track and guessing fuel starve vs electric + heat.
Once you have 100% eliminated fuel delivery then start on electrics.

I say this based on going the random swap out electric parts only to find my misfire was due to not enough fuel.

Actually it probably was both a marginally coil and boiling coolant because it was fixed when we fixed both issues (radiator cap had a failed seal...replaced with the one from my truck and system refilled fixed it).  That said, we might find out next race.

Edit:  Doh!  Nevermind.  Is your fuel delivery issue fixed for Barber now?

  • Dave
  • Dave
  • Effluent
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  • From: PGH PA
  • Registered: 2009-10-09
  • Posts: 993

Re: Parts swapping verses diagnostics

I have a simple theory, if you can still do laps without causing too much damage fix it later. I usually race in class c so as long as I'm on the track longer than the rest of the field I should win. But then again, sometimes we have to put me in the car to diagnose it to see if we should continue of pull it in to fix. Then I change everything I think could cause the problem. The last race I guessed wrong and got a dnf. BYW when I took a look at the shop I fixed it in about 10 minutes. No starts are way easier than runs bad at times. I sure hope the runs bad at time is all better now.

It Ain't My Fault

Dave's Website

  • From: St Louis
  • Registered: 2011-10-20
  • Posts: 2,611

Re: Parts swapping verses diagnostics

OnkelUdo wrote:
Brett85p wrote:

Gauges, plenty of gauges!
Basic O2 at least or better yet wideband O2, this will at least confirm or eliminate fuel mixture as a cause. Also fuel pressure gauge is nice if you are missing on track and guessing fuel starve vs electric + heat.
Once you have 100% eliminated fuel delivery then start on electrics.

I say this based on going the random swap out electric parts only to find my misfire was due to not enough fuel.

Edit:  Doh!  Nevermind.  Is your fuel delivery issue fixed for Barber now?

Yes bigger injectors and some HP Tuners adjusting fixed it. Engine is currently out just got a new timing chain and that was just in time, lucky it lasted the Joliet race with straight valves!  Taking the opportunity to build a better trans tunnel and tidy up a few things that didn't get done right with the initial swap.

Apocalyptic Racing - Occupy Pit Lane racing
Racing the "Toylet" Toyota Celica powered by Chevrolet Ecotec.
24x Loser with the Celica. 16x loser in other fine machines
Overall winner Gingerman 2019

Brett85p's Website

  • From: Danville CA
  • Registered: 2011-04-24
  • Posts: 556

Re: Parts swapping verses diagnostics

If you don't have all the info you need to make a definitive diagnosis, always adjust/inspect/swap the cheapest part first.

Case in point- a leaky master cylinder might just be an improperly sealing/seating master cylinder reservoir cap. One takes 5 seconds to swap, the other about :45-:60 if you hustle and everything goes smoothly. We learned this the hard way at Sonoma in December,

Bert One - 1981 Volvo 262c Bertone | Ernie - 1990 Nissan 240SX
  • Registered: 2011-10-14
  • Posts: 4,167

Re: Parts swapping verses diagnostics

Brett85p wrote:

Engine is currently out just got a new timing chain and that was just in time....

Hopefully not from the guy on eBay who sold you your Toyota valves.

  • From: St Louis
  • Registered: 2011-10-20
  • Posts: 2,611

Re: Parts swapping verses diagnostics

OnkelUdo wrote:
Brett85p wrote:

Engine is currently out just got a new timing chain and that was just in time....

Hopefully not from the guy on eBay who sold you your Toyota valves.

Nothing but the best (second cheapest listed because it has to be better quality) from Rock Auto.

Apocalyptic Racing - Occupy Pit Lane racing
Racing the "Toylet" Toyota Celica powered by Chevrolet Ecotec.
24x Loser with the Celica. 16x loser in other fine machines
Overall winner Gingerman 2019

Brett85p's Website

  • From: Menomonee Falls, WI
  • Registered: 2013-02-12
  • Posts: 300

Re: Parts swapping verses diagnostics

Because I've mentioned this at the track and you're still doing it... wink

You meant to type
On Board Diagnostics

but instead you typed:
Old Dirty Bastard

-Nathan - Team Captain, Priority Fail Racing
1997 Golf GTI VR6 Mid Engine

the shaolin's Website

10 Reply by OnkelUdo (edited by OnkelUdo 2015-12-30 05:32 PM)

  • Registered: 2011-10-14
  • Posts: 4,167

Re: Parts swapping verses diagnostics

the shaolin wrote:

Because I've mentioned this at the track and you're still doing it... wink

You meant to type
On Board Diagnostics

but instead you typed:
Old Dirty Bastard

Well, both are appropriate if I am the one you are talking about...cause you know I would so still steal that tasty wife of yours even though I like and respect you so much.

  • From: Houston, TX
  • Registered: 2012-06-21
  • Posts: 1,608

Re: Parts swapping verses diagnostics

OnkelUdo wrote:

So...at the track, I throw parts at it until it works until the race day ends.  Online and in my garage, I diagnose, test and often repair.  Where do you fall in the spectrum.

Basically this, but it really varies based on the problem. It helps having multiple people on the team with overlapping specialties so one person (usually me) doesn't go too far down a rabbit hole.

The biggest lesson my team learned at our second race was this: take nothing for granted and be very careful about your assumptions. Just because you "fixed" something before the race doesn't mean you made it any better. Another lesson we learned later was "if stumped, go find Anton and ask him." big_smile

Escape Velocity Racing
  • Registered: 2009-07-19
  • Posts: 1,585

Re: Parts swapping verses diagnostics

It depends on the car. With the Duff car; we've been racing it since '09, and I'm the primary mechanic, so I almost always know what's broken as soon as it exhibits a symptom. At the track I will throw parts at it, but when I get it back to the shop I try to figure out what caused the problem.

With other vehicles I do diagnostics, generally with the shop manual, although there are exceptions. The rallycross Sentra wouldn't shift out of 1st (3-speed automatic transmission), so I opened it up, cleaned the valve body, adjusted the band, replaced the valve body with one out of an old Pulsar, and then finally bought the factory transmission manual for $35 online. On step 5 of the diagnostics procedure I found the very easily replaced $6 plastic gear on the regulator that was missing some teeth and fixed the problem.

Everybody grab your brooms, it's shenanigans!