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Author Topic: seemingly random bogging at low rpm, high throttle (mk1)  (Read 533 times)

Offline SC S12

Been a long time since I posted.
I am once again asking for your support...

As stated I have a strange random loss of power, specifically in the 2000 to 3400 or so range. Sometimes it just falls on its face to the point of full throttle barely making it accelerate any faster.

But its not all the time, nor is it closed/ open loop Dependant.

Things ive checked : fuel system. New pump/check valve (ecu said it had a fault) new fuel filter. After warmed up fuel line pressure is a steady 37 or so psi, which jives with the manual. Keeps around 25psi in line when cut off with no leakdown.

Rotor checks out as fine via multimeter. Cap checks out as good via multimeter. Wires are about 3yr old, coils are about 2yr old, and still work independent, I never bridged the exhaust to the intake side. Pulled a few plugs randomly and they look good as well, no buildups. Ordered new rotors in case, have a stock pile of caps.

O2 sensor is relatively new, and reads out correctly (around .565v at idle with some light bouncing, nothing major)

Things I replaced and am not sure about : all vacuum lines were replaced about 3 months ago. I did not keep the filter/flow reducer sections that go from the throttle body to the TVV. Could this be causing it to suck too much advance and stumble? I had to drop the idle like 200rpm after removing these sections. Else it would idle around 1000 when you hoocked the vac line back to the dizzy.

Things I'm not sure on, haven't touched :
Chts : replaced when I got the car like 5 years ago, but seems fine.
Tvv : replaced a few years ago, did not ever let the vac advance work. New one never seems to kill the vac advance in my current testing(almost like its stuck open) ordered a new one, have yet to install.

Considered bad fuel, but it has persisted across a few fill ups of varying grades.

Another oddity : the exhaust side coil never stops firing. On my mk1.5 it cuts out at 4000 rpm or so. I drove the mk1 I'm messing with on both, e only, and I only coils all the way to redline without issue. Seems odd. Book says that they both fire any time chts says 59f or less, but other temperature related things are working (fuel injector cut off)

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Beam me up Scotty,
There is no intelligent life down here


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Offline kelso840

Re: seemingly random bogging at low rpm, high throttle (mk1)
« Reply #1 on: 02:16:24 PM / 11-Mar-20 »
Try swapping the lead wires between ignition and exhaust coils and see if you notice any change. Also check to make sure there's no corrosion building up on the ground wires coming off the coils.

I haven't had a CA for a long time but the coils always seemed to be a source of funny business.
Infrequently driving an s12.

Offline BOSSMAN

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Re: seemingly random bogging at low rpm, high throttle (mk1)
« Reply #2 on: 02:44:03 PM / 11-Mar-20 »
I had similar issues with bogging at high rpm with my turbo.  One time was a cut with my primary coil wire to cap, wires were three months old at that time, on the second time it was reversed primary plug wires between cylinder 1 and 2 after I put new plugs in.  Like you said the secondary coil should stop work when the engine go pass 4000 rpm under heavy load so in both cases there was a problem with my primary ignition side when my secondary side stops working.  One member I helped out a long time ago with bogging turned out to be his newer fuel pressure regulator he just installed.  I'm not sure how the NA pressure regulator works but for the turbo under boost (no vacuum) the regulator will open to let more fuel in but the guy put a regulator from a CA18det and that one works in reverse.  The newer regulators are place in the outlet side of the fuel rail controlling the flow going out whereas our older cars have the regulator in the inlet side of the rail controlling the flow going in.

Post Merge
PS: see kelso840's reply and double check all the ignition wiring.
« Last Edit: 02:46:30 PM / 11-Mar-20 by BOSSMAN »
86 CA18ET....owner since 1990.
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Offline SC S12

Re: seemingly random bogging at low rpm, high throttle (mk1)
« Reply #3 on: 11:01:55 PM / 11-Mar-20 »
some update with a mostly fixed issue:

on a whim, I kept all the old vac lines i took off in the past.
spliced the flow restrictors from the two manifold vacuum lines into my new lines. Re-adjusted idle again and re-timed for the new idle speed.

As it turns out, at least on this car, If you dont have those flow restrictions in feed lines for the TVV, when you set the based time to 4* BTDC, when you hook the vacuum line back to the dizzy your idle time is around 22*btdc, Any slight blip of the throttle makes the timing marks totally vanish from sight like it just maxes out the vacuum advance instantly. Though it never sounded like it, i suppose it was misfiring or pinging like hell when was getting no throttle response.

with those back in the loop, after you set the base time to 4*btdc and hook it back up to the dizzy you are sitting around 14-15 BTDC. I still have a small miss or stumble here or there but nothing like it was.

Gonna just do the standard coil/rotor/wires/plugs and a Italian tune up and see where it falls after the weekend.

need to get this turd ready for radwood
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Offline SC S12

Re: seemingly random bogging at low rpm, high throttle (mk1)
« Reply #4 on: 07:12:35 AM / 01-Aug-20 »
New updates.

Did Timing Belt and water pump+thermostat (stock temp), front main, and front cam seals, plugs +wires + dizzy cap and rotor like 2 months back.

Drove the car roughly 1k miles since, And it was doing great. Too good honestly. 28MPG first tank with AC on. Tanks on this last Trip where 27 (73MPH) and 36MPG return (67mph)

At the end of a 450 mile road trip where it functioned perfectly, it started doing this crap again about 3 miles from my house so badly that foot to the floor in 3rd gear it would slow down.


Engine takes FOREVER to warm up (like 15 minutes idle) It only reaches 1 bar into the operating range. Suspect timing is just too far advanced. I don't suspect that its a thermostat as its a new one.

Very low/rough idle when cold. Sometimes nearly stalls when clutching coming to a stop. Ocassional misfire at 40-50MPG in high gear at moderate acceleration. Cannot hear if its pinging.

Base Timing checks out perfectly at 4-5* BTDC. HOWEVER Vacuum advance at idle is about 22* again, instead of 14-16*. The lightest blip of the throttle pushes the advance i'm Guessing all the way to max (maybe 40* ?)

Will replace TVV and Update later today.
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Offline weitrhino

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Re: seemingly random bogging at low rpm, high throttle (mk1)
« Reply #5 on: 02:41:42 PM / 01-Aug-20 »
Pulled any ECU codes?

Offline SC S12

Re: seemingly random bogging at low rpm, high throttle (mk1)
« Reply #6 on: 07:42:00 PM / 07-Aug-20 »
Pulled any ECU codes?
Only one is the fuel pump (wrong sending unit so it always is in the current/history and my fuel gauge does not work properly, but the pump itself is a new JECS unit)

I replaced the TVV and it helped out a lot, especially in the low end and just off idle. No More stalling Coming to a stop.

I just barely touched the nipples and the whole top of the TVV shot off like a rocket. I'm Not sure why this thing failed, it was only 2-3 years old. Found all the springs and valves and it looks like it was dragging on the little pin too hard on one side, And I guess was stuck or the housing was cracked.

After Thinking about it, I just replaced the Primary and secondary coils and that seemed to fix the high end breakup. They were both about 1000Ohm too High on the secondary circuit.

Its back to running well, and running at temp for now. It seems odd that id have two failures at the same time, but... who knows.
Beam me up Scotty,
There is no intelligent life down here


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Offline weitrhino

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Re: seemingly random bogging at low rpm, high throttle (mk1)
« Reply #7 on: Yesterday at 09:55:03 PM »
Good diagnosing. With a 35 year old car I've tried to stop thinking in terms of a single problem - single fix. It's not always so easy though.