20V Blacktop intermittent misfire

GZE_RA21
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20V Blacktop intermittent misfire

Postby GZE_RA21 » Mon Aug 21, 2017 2:01 pm

Greetings once again C4AG!

So I finished the bulk work of getting my 20VBT swap done in my 85 GTS hatch, just ironing out the little kinks and tying up a few loose ends, good news is... it drives! Surprisingly well at that! Hands down one of the easiest swaps I've done, random issues wise that is. I did notice something strange that concerned me though and was wondering if any of you had an idea what it could be?

The trouble with this issue is that the engine it runs absolutely flawless, the issue only occurs around 4-4.5k rpm in 5th at highway speeds cruising consistently for an extended time (5min +) and when it misses, it is an abrupt few shakes. It scares the hell outta me with how random it is, which as an involuntary reaction, causes me to let off the gas. As soon as I do the shaking stops and it cruises like nothing happend. When it happens it is totally random, not worse from going over bumps in the road, flat ground, incline, nothing seems to trigger it more or less often, and like I said it's about every 5-10 min, but not constantly every time I take it on the freeway, but when it does start happening it usually happens a few times in the same trip....

I'm thinking it's most likely ignition, because it revs and has good power everywhere, just like a freshly rebuilt engine should. Not ruling out fuel but it seems strange that it's a problem only at a given rpm/speed while cruising in 5th.

Just to list potrntially pertinent things that have been replaced...
New OEM distributor cap/rotor
New denso spark plugs
New denso spark plug wires
Confirmed all plug wires are fully seated on the plugs
Timing was set per 20vBT FSM
OEM 20v BT manual ecu
Distributor is in good condition
Idle is fine
Starts up and revs perfectly fine, not laggy at all
New fuel filter
Injectors all functional

I might be missing a few things but that's about where I'm at, any ideas fellas? Many thanks in advance! :)

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jondee86
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Re: 20V Blacktop intermittent misfire

Postby jondee86 » Tue Aug 22, 2017 2:41 pm

What heat range Denso plug are you running, and how do they look if you pull them
soon after a highway run ? What kind of outdoor temperature do you have while driving
at your location (days when you have the misfire happening) ?

Cheers... jondee86
Give a person a fish, they eat for a day. Suggest they search before posting, and they learn a skill for a lifetime.

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s24a
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Re: 20V Blacktop intermittent misfire

Postby s24a » Tue Aug 22, 2017 4:56 pm

Are you running an O2 sensor, and if so is it new as well?

If not, the vehicle may be finding lean spots in the volumetric efficiency maps.
"When the going gets, wierd, the Wierd turn Pro" -- Hunter S. Thompson

GZE_RA21
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Re: 20V Blacktop intermittent misfire

Postby GZE_RA21 » Tue Aug 22, 2017 8:34 pm

jondee86 wrote:What heat range Denso plug are you running, and how do they look if you pull them
soon after a highway run ? What kind of outdoor temperature do you have while driving
at your location (days when you have the misfire happening) ?

Cheers... jondee86



unsure of the specific heat range, I just got standard plugs. I'll have to get back to you in a bit with how they look. Ambient temps while driving have been very mild for August, 75-85 degrees average.

s24a wrote:Are you running an O2 sensor, and if so is it new as well?

If not, the vehicle may be finding lean spots in the volumetric efficiency maps.


Yes, a brand new denso o2 sensor also (knew I missed something) although not specific for the blacktop, it has the same 4 pin connector, I believe it was for a mid 90's Tacoma 4 cylinder IIRC.

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jondee86
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Re: 20V Blacktop intermittent misfire

Postby jondee86 » Wed Aug 23, 2017 12:23 am

OK... I was just wondering if your plugs might be overheating and causing a little
bit of detonation... just enough to have the ECU knock back the timing ? As said
above, if the engine is running a little lean (maybe due to open throttles ?) then it
will likely generate higher combustion temperatures when running at constant speed
on the highway highway with light throttle.

You could also check the colour of your tailpipe after a highway run. If it runs a light
'tan colour then it is likely you are a bit lean. Depending on what the plugs and pipe
look like, it might be worth trying a set of standard plugs in a cooler heat range...
something like NGK BKR7E-11.

Cheers... jondee86
Give a person a fish, they eat for a day. Suggest they search before posting, and they learn a skill for a lifetime.

GZE_RA21
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Re: 20V Blacktop intermittent misfire

Postby GZE_RA21 » Thu Aug 24, 2017 12:39 pm

jondee86 wrote:OK... I was just wondering if your plugs might be overheating and causing a little
bit of detonation... just enough to have the ECU knock back the timing ? As said
above, if the engine is running a little lean (maybe due to open throttles ?) then it
will likely generate higher combustion temperatures when running at constant speed
on the highway highway with light throttle.

You could also check the colour of your tailpipe after a highway run. If it runs a light
'tan colour then it is likely you are a bit lean. Depending on what the plugs and pipe
look like, it might be worth trying a set of standard plugs in a cooler heat range...
something like NGK BKR7E-11.

Cheers... jondee86



Hmmm this is just a guess, but if the knock sensor was pulling timing... I don't think the car would shake as abruptly and with as much force as it does, wouldn't it just be a loss in power when it retards? The feel of the car when it starts shaking is almost as if it loses one cylinder for a second or two until I let off the gas. Like I said, it's so abrupt and dramatic that it literally startles me enough to let off the gas involuntarily.

At any rate, I did pull the plugs, they are NGK not denso like I stated in the first post, same with the plug wires as well in fact. I'm no expert, but they look pretty normal for plugs with 300 miles on them, no brown deposits out the tailpipe either, just black soot from being a bit rich when you really get on the throttle. All this leads me to believe it's not running lean.
Having trouble getting flickr to work for some reason now, but I'm posting links to the pictures....
https://www.flickr.com/photos/138619479 ... ed-public/


On a side note, I'm committing 20v blasphemy by not running open throttles :o I found it to my benefit to modify the stock air box to work in RWD format for several reasons.

https://www.flickr.com/photos/138619479 ... ed-public/

Could the way I have the spark plug wires routed possibly have anything to do with my stumbling issue? I have them spaced out pretty well I thought, but then again they aren't the stock 20v wires either.

https://www.flickr.com/photos/138619479 ... ed-public/

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jondee86
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Re: 20V Blacktop intermittent misfire

Postby jondee86 » Thu Aug 24, 2017 6:04 pm

Finding an intermittent fault such as you have described, is one of the more difficult
jobs you can have to do with a vehicle. The circumstances under which it occurs can
sometimes be the only clue you have to try and troubleshoot.

For the ignition system the most likely time for things to break down is when the engine
is under heavy load. For example, accelerating up a long uphill grade in a high gear with
the engine near peak torque. This demands the highest output from the ignition system,
and will pinpoint worn plugs, a weak coil or carbon tracking.

Pretty much the same for the fuel system. If the fuel demand is high but the pump is
weak, filter clogged or FPR failed, then it will be load related.

If the problem only occurs after extended high speed (but not high load) running, then
you need to look at what particular conditions the engine sees under those conditions.
Without a WBO2 gauge to check the cruise AFR, it is possible that the factory ECU is
running at stoich or even a little leaner in the interests of maximum fuel economy. It
may also be dialling in a bit more advance if the engine is only lightly loaded.

That is a condition that will generate higher than usual combustion temperatures, and
can get a spark plug hot enough to cause detonation. I don't know enough about reading
plugs to be able to offer a definite opinion on yours. But from what I see browsing the
interwebz, I'd suggest you check your ignition timing. And for the sake of the small cost
of a set of standard plugs (no need for irridiums) I'd still suggest trying a set of colder
plugs (heat range 7 for NGK).

Cheers... jondee86
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