Revs cut, backfires on 15° normal mode, 20° is ok

anyzunstudios
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Joined: Wed Feb 06, 2019 11:31 pm

Revs cut, backfires on 15° normal mode, 20° is ok

Postby anyzunstudios » Fri Apr 01, 2022 4:22 am

Hi guys,

Sorry for that weird description :mrgreen: :mrgreen:

Well, my problem is that I am having issues with the timing and rev cuts around 1.8k-2.5k rpms.

Also, I can’t set the timing marks fine because on diagnose mode, when I put engine in 10°BTDC engine backfires and sounds horrible, if you try to press throttle sounds like knocking and hesitates like engine is not getting any kind of fuel, so I have to set timing on diagnose mode in 15°BTDC on diagnose mode, so in normal mode it’s like 20°BTDC.

I might think there’s a leak somewhere, because idling in quite high for me (1.3k-1.2k rpms) after idling a lot of time and fans were on for a long period. On cold startup sounds like a vacuum over a carpet :mrgreen: but after couple of minutes idle goes down and I can barely listen to it. No trouble codes on EFI. TPS sensor changed. Fuel pump were changed like one or two years ago. I tested the pressure on the rail and there’s more pressure idling than the minimun required, when rise the rpms it barely goes up but there’s more pressure that toyota says for full throttle, but in a fast throttling engine sounds like knocking and hesitates for a second and then revs up.

I changed the ignition module (I have 2 old ignition modules) and also swapped the coils. Nothing. I bought a NGK coil for previous Celica 2.0 engine that is like 4age but about 0.5mm narrower and 1cm larger. Nothing. All spark wires were changed and they are new (not original). Distributor cap is new and rotor is new too.

Alternator regulator was changed and outputs 14.5v. Ground and positive wires were improved. The only thing that seems not too good is the battery that is a 60Ah batt when toyota recommends 45Ah batt and I have a battery checker on the battery itself (its a bluetooth module) and seems a bit worm out 54% or so.

What I notice is, when engine is warm up and in operative mode 80°C and above, I have a device that reads values from stock EFI (toyocom) and I can see that when EFI voltage is lower than 13.3v it starts to do that weird rev cut in the range of 1.8k-2.5k rpms.

While driving everything seems fine, but on a traffic light, when I start to drive again I have that annoying problem.

Any ideas?

Thanks in advance!

Regards,
Julio.

totta crolla
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Re: Revs cut, backfires on 15° normal mode, 20° is ok

Postby totta crolla » Fri Apr 01, 2022 10:26 am

A basic test.
Make sure that TDC is actually where the crank pulley says that it is. You can do this by putting a long screwdriver down a plug hole to sit on top of the piston.
It is quite common for the crank pulley outer ring (with the timing marks on) to slip.

sirdeuce
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Re: Revs cut, backfires on 15° normal mode, 20° is ok

Postby sirdeuce » Fri Apr 01, 2022 11:53 am

Yep, verify TDC on #1 cylinder AND in the distributor. If it's not right the ECU will think #2 or #4 cylinder is #1 and that can throw things off.
Then there are issues with the igniter and Ne and Ge sensors. If the ignitor is going out it won't send the proper signal to the ECU. If the pick-up coils in the distributor are going out weak or erratic signals to the ignitor and ECU can cause issues. Could be the ECU.

Now the fun begins! Pull out ye olde multimeter and start testing.
One shot, one kill.

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mad_86
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Re: Revs cut, backfires on 15° normal mode, 20° is ok

Postby mad_86 » Mon Oct 24, 2022 10:12 pm

Hey sirduce

Any update or fix on this problem you were having , I’m having similar problems with a friends corolla

Corey20v
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Re: Revs cut, backfires on 15° normal mode, 20° is ok

Postby Corey20v » Wed Oct 26, 2022 10:16 am

In general, things I've observed w other older cars and may help:
Set engine to correct RPM idle & in diagnostics mode if applicable.. dont bother adjusting timing with a timing light if you cant get it to idle low/high/consistent enough. Chase vacuum leaks, idle air control valves, bad throttle body seals etc first.
Tbh on any old car now, i will just pull the 1st spark plug and set the engine to TDC with a long screwdriver, and verify the timing mark is even still correct on the pulley... And paint the marks too. My wifes 93 miata timing mark was ~35deg off because the pulley slipped on the damper, moving the marks... So i painted a new mark to temporarily time it while i find a new pulley. It felt perfect after.
Timing is right when it is at the correct degree and at the correct rpm at the same time.
1986 SR5->20V ST (WIP!)
Other toys: 1993 S13 Coupe, 1993 NA6 Miata, 1996 Z32 300ZX