Check Engine On Off On Off - Repeat

2000 4Runner V6 Manual 389K Miles - Well Maintained

My 4Runner very suddenly started running rough, then Check Engine started flashing, then solid. Next morning, ran smooth as usual and Check Engine eventually went off on its own. Next day, suddenly rough again, Check Engine on, later smooth, Check Engine off. And repeat almost daily. I’ve gassed up at three different locations just in case. Also added BG44K.

Managed to get codes when Check Engine was still on once - P300, and other PXX codes - cylinder misfires.

My go-to shop replaced spark plugs, wires and coils (they were all overdue anyway). Next day, same problem. An engine compression check showed two cylinders below spec compression. Recommendation - engine rebuild - I laughed at the price.

I understand compression loss - it’s an old truck - but the Check Engine symptom started abruptly and self corrects, and I don’t think the compression issue is suddenly fixing itself, then dropping again. Anyone familiar with this problem?

Flashing CEL means serious misfiring & to stop driving immediately, otherwise risk engine damage. Misfiring, common causes are plugs, wires, coils, ignition module, crank & cam positions sensors, valve timing, rings, pistons. You’ve already replaced the first three, so you could continue on w/replacing the ignition module next. I’m guessing however you’re having problems w/the valve timing. Maybe a faulty variable valve timing actuator or failing timing chain mechanism. Suggest to post the measured compression in all 6 cylinders for more ideas here. Any other diagnostic codes other than the p03XX’s? If so, post them too. 390K miles? that’s pretty good. Not a record, but approaching one.

My bet is that the compression in one or both of those cylinders is right on the edge of causing misfires.

I’d guess one or more of your fuel injectors are sticking open longer than they should.

The mechanic needs to scan your truck while it exhibits this otherwise even he is guessing.


I would try decarbonizing the intake valves.


Low compression can cause spark plugs to misfire and those can be a hit and miss affair.

Just curious, but what kind of low compression numbers are you talking about?

As a follow-on, the OP could buy or borrow a scanner and attach it to the OBD-II port. When the truck starts running rough the will have a record for the mechanic to review.

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There’s quite a list of things that can cause misfires. A poor ground connection between the engine and the frame or between the engine and the battery directly can be the culprit. With the high mileage and a sudden onset it seems more likely that it’s an electrical component, but that’s really just guessing.

If you are going to be paying a mechanic it’s time to evaluate what your long term plans are for this vehicle.

Thanks for all the input! Regarding compression, here’s what I got:

1 - 155 PSI 2 - 165 PSI
3 - 125 PSI 4 - 130 PSI
5 - 110 PSI 6 - 155 PSI

1 - 5-10% 2 - 5-10%
3 - 30% 4 - 5-10%
5 - 30% 6 - 5-10%

Bought a Bluetooth code reader and check it when Check Engine comes on - almost always P300 (cylinder misfire), sometimes I get specific cylinders. Once yesterday just a P0135.

Today, started up fine, Check Engine was off, drove 50 highway miles without issue. Weird.

Had one shop suggest some kind of BG engine treatment process they do in the shop for a few hundred bucks.

110-130 psi are definitely too low. If you just had one cylinder at 130, you might be able to get away with it. But half of the cylinders at 130 or below, I expect that’s the cause of your engine problems. The leak-down test suggest the problem might only be the head gasket, with the valves and piston rings still ok. Removing the cylinder heads for a look-see seems to be the next step. I wouldn’t suggest any miracle engine treatments as those – while they may help temporarily – may cause other engine problems over time and make the head gasket job more difficult. Hopefully the p0135 will go away with new head gasket. If not, that’s likely either a faulty o2 sensor, wiring short- or open- circuit, or corroded connector.

Blown head gasket. Two low compression cylinders next to each other confirms it. Leaks coolant in to the cylinders and then it misfires on those for a while until the coolant is cleared out.