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Matt, Mechanical Engineer
Category: Toyota
Satisfied Customers: 20385
Experience:  BEng, Repaired my own cars and others for 20 years, worked in Formula 1 for 10 years
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Would accidently using 87 octane(instead of premium) in a

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Would accidently using 87 octane(instead of premium) in a 2000 Celica GTS cause a code P0302 random misfire?

Hi! Welcome to JustAnswer. Thanks for coming! I'm Ron Z and I'll be helping you with your question today.

Was the P0302 the only code set?

Customer: replied 9 days ago.
First time through emissions it showed P0300 also. Have replaced irridium plugs, had coil packs,compression,and injectors checked out by mechanic. The CEL is VERY dim and hard to see unless it is night. We are fairly sure that it wasn't on when purchased, but realized the car required premium after 3-4 fillups(sticker was missing) Was hoping a few cycles of premium would reset. Traveled 40 + miles interstate after reset and light stayed off. Less than a mile after restart, got CEL again.


if you've been using the car hard and its hot weather then yes 87 octane can create issues

not so much with misfires but with detonation / pinking which can damage the engine is left unattended

I'd check that the ignition timing is set correctly as the next step, its not adjustable but its well worth checking it with a timing light to ensure its within the 10-18 degrees its meant to be at hot idle

Check the resistance of the spark plug leads ( 13-15Kohm / metre) and replace if any doubt or high resistance

You can also try spraying some water on the coil pack with the engine running. It is possible the coil is faulty and causing the misfire. If you see sparks arcing with the water, then replace the coil.

I'd check the condition of the injector plugs / connections as this could be a lean misfire if an injector isn't firing properly - the best way to check this is with a 'noid' light

Then check all the connections to the engine sensors one at a time, remove each one and examine for signs of corrosion or damage and a squirt of contact cleaner ( not WD40) will also help.

This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and leans the engine out causing rough running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas around each joint in turn. If the engine rev's up you've found your leak.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Customer: replied 9 days ago.
Car seemed to run just as well on Regular, no pinging.Still maybe a little sluggish at less than 3k rpm.Mechanic did the noid light and fuel pressure test, swapped coil packs (still came up Cylinder 2).If improper gas was problem, do you think this will correct after a few tanks? Not familiar with "The best way to locate a leak is to have the engine running and warm and then spray lighter gas around each joint in turn. If the engine rev's up you've found your leak." Can you elaborate? Lighter fluid, joints? Thanks


if you use brake cleaner or propane/ butane gas( lighter gas) and spray it around the intake system then the engine pulls it in as fuel and will rev up slightly

another way of detecting vacuum leaks is to do a smoke test

Customer: replied 8 days ago.
My mechanic has now checked everything you mentioned, but suggested a "leak down test' but he doesn't have the equipment. He knows I don't want to take a $2500 car to the dealer. Since it is P302 code consistently, do you think a valve might be sticking? A correction to my earlier post, low power/ rough under "2K rpm".


a leak down test is a more sophisticated compression test really and if you guy has a compression test gauge then its worth doing that as this too will show if a valve is sticking

If you test on all cylinders as they should all be within 10-15psi of each other

you just have to remove all the spark plugs and spin the engine over on the starter with the throttle fully open and the gauge fitted to the 1st cylinder - make a note of the reading, remove the gauge and move onto the next cylinder - takes about 20 mins

Customer: replied 8 days ago.
He did a compression test, cylinder 2 @ 120, other 3 had 150. Car runs good above 2k rpm. Would lower #s on 2 cause that misfire? It will some times go for a bit before throwing code, bit I have to take care of it since we have emissions testing.


OK that much lower can give issues but its not that bad either

if the test was repeated with a squirt of engine oil into each bore and see if the pressure jumps up

if it does then the issue is with piston rings, if the reading is the same then its a valve not sealing

Customer: replied 6 days ago.
Took it to another Mechanic, did leak down test and it's cylinder 2, not the valve tripping the CEL. Still has 130 compression. If I just keep check on the oil, do you think it'll be okay to keep driving? Thanks for your help!


yes if you keep an eye on the oil level ( as its probably using a bit) then you can keep driving it

Matt, Mechanical Engineer
Category: Toyota
Satisfied Customers: 20385
Experience: BEng, Repaired my own cars and others for 20 years, worked in Formula 1 for 10 years
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