Engine knocking/shaking upon starting, but not every time ??

I got the car back from the mechanic yesterday. They cleaned out the EGR valve, and the engine sounds like it is running fine. On cold start up, there was a *tiny* shudder, but nothing else on subsequent restarts of the ICE during regular driving. I even did some tests on it sitting in a parking lot, stopping, waiting for the engine to shut down, and leaving it off for an extended period of time before accelerating again. Essentially, I was recreating the sorts of conditions in which the shuddering happened- and doing the sort of test driving I had done to successfully recreate the problem. Nothing further happened.

The problem now, is that I have a check engine light, which is showing misfires with cylinders 1 and 3. The mechanic was not able to clear the light at all. It persisted.

Due to a variety of issues, it is best for me to just take the car back to the mechanic Monday morning, instead of leaving it with them today. Hopefully, we will figure something out rapidly. The nice thing is, my mechanic seems committed to getting It right without charging me anything further, after last week's initial replacement of spark plugs and a cylinder one coil was not the problem.

 

This has all ended well. I took the car back to the mechanic today, and it was ready within a few hours, on a day when they were fairly swamped.

It turns out, one of the new guys was doing part of the work on the EGR valve cleaning, and he hadn't properly reattached an air flow sensor.

The issues have been going on for nearly two weeks, with four visits to the mechanic. Fortunately, all he has charged me after all that, was for the initial spark plug changes and new coil.

 

Apparently, I spoke too soon. I had to leave the car in their overflow lot, and pick it up this evening. When I picked it up, it was in the sixties outside, and my temperature setting for the inside of the car was in the seventies. The heat came on, but it never got warm. In addition, I have a check engine light again.

I went out to the car, and cranked up the cabin temperature to 85, and let the car run for a bit. It never got warm. Usually when the heater is on, the engine runs a bit to maintain the temperature. The engine never came on.

 

I think your mechanics have dropped the ball. Doing the EGR cleaning at 195K, with a P0301 code, is not going to turn back the clock. I'd get someone to do a leak-down test, see where the head gasket is at.

 

I'm starting to think the same way. I had mentioned the head gasket before the EGR cleaning was done. He seemed to think it was not the head gasket.

I'm not entirely sure the science behind why the head gasket would *not* cause the check engine light when it was warm out and I drove it around after it was ready, then *would* cause the check engine light when it was cold out, and I turned on the heater. Maybe something to do with the coolant in the heater core.

At least it seems the fault with the head gasket, if that is it, is not bad enough that it shows any other symptoms at all.

 

Have you had the car since new? A lot of the third gen and v's get resold around 150K, and surprise, surprise: stop-leak product in the engine coolant.

 

No. I have had it since it had about 95k miles on it. This is the first time I have had any sort of problems whatsoever.

When they did the EGR cleaning, all was well, except for an air flow sensor one of the younger, newer mechanics had not reinstalled correctly. When that got fixed, there was this odd behavior. As I mentioned, no check engine light when I drove it around initially after it was ready. Everything seemed perfect. It was just on the way home last night that the heater wouldn't heat, and I got the light again. The engine sounds like it is running perfectly, and I can't tell there is anything else amiss at all.

 

Is engine coolant level stable? I appreciate with recent EGR cleaning it's likely just been refilled, but keep an eye on it. The "traditional" head gasket failure point on 3rd gen is between cyl's one and two, and (with neglected EGR) they tend to let go between 150K and 200K. What happens is coolant starts leaking into cylinder, causes knocking at subsequent start-up, since the coolant is uncompressible. Procratinate long enough and a couple of things can happen: the damper between engine and transaxle breaks, or a piston rod bends.

 

The coolant is perfectly stable.

The last misfire codes, which resulted from the airflow sensor problem were for cylinders 1 and 3. Still waiting to hear back from the mechanic on the code that popped up last night.

 

Hang onto the old coils.

 

Only one was replaced, and at this point, it's probably two weeks too late to hang on to the original one.

 

The replacement of that coil did not solve the problem, as you may have read way farther back in this thread. That was what led to doing the EGR cleaning.

 

So let's go back and look more broadly at things.

EGR stands for Exhaust Gas Recirculation and it does just that - it is a system that allows some exhaust gases to enter the intake alongside air. This was done to lower emissions, but not to "reburn" incomplete combustion as many people assume.

Exhaust gas recirculation was actually done because it lowered combustion temperatures, especially when running lean (lean means using relatively little fuel for a given amount of air). Running a bit lean is good for fuel economy and hydrocarbon emissions, but the higher combustion temperatures accelerate chemical reactions that lead to various nitrogen oxides (NO, NO2, N2O, etc - all lumped into the category of "NOx"). These contribute to acid rain, smog, and ozone. Adding an inert gas to the combustion chamber lowers temperatures, greatly reducing NOx emissions. And it just so happens that your engine has a steady supply of mostly inert gas in the form of exhaust.

Toyota went even further with EGR and uses it more heavily to reduce energy losses due to pumping inefficiency. You see, gasoline engines use a throttle valve to adjust how much air enters the engine. The engine then injects the corresponding amount of fuel for that amount of air, and the driver is rewarded with the precise amount of power he desired. But when you're not flooring the accelerator, your throttle is partially blocking the intake which adds restriction to the air. It is like trying to breathe through a straw. This restriction requires energy to overcome, which ultimately comes from the fuel you burn.

So in its quest for maximum fuel efficiency, Toyota decided to use the EGR a lot more aggressively in order to keep the throttle body open more while still delivering only the modest amount of power demanded at that moment. A faulty EGR system in most cars increases tailpipe emissions and throws a warning light, but the driver otherwise probably would never know. A faulty EGR system on the Prius causes much more substantial driveability issues.

So with all that being said, why might a Prius EGR issue cause a large start-up problem that seems to go away when warm? The most likely reason is carbon fouling causes a sticky EGR valve to fail to quickly reach the precise position demanded. As the car warms up, some of this stickiness goes away. But it is probably still causing problems - just not as noticable if you aren't religiously monitoring mpg (at least until it contributes to a blown head gasket).

It might also be that the EGR problem is electrical. Perhaps when cold (and/or damp) one of the connections isn't reliable but that connection improves at high temperatures. Sometimes cleaning an EGR valve doesn't help - sometimes it hss to be replaced...

 

If someone used a coolant sealer as a "solution" it can cause blocked heater and thermostat passages. In most cases a sealer does temporarily eliminate the cold start rattles.

You are right to question the egr hypothesis often offered around here for cold start rattles. The reality is the egr system does clog up the egr cooler because of a poor design pulling exhaust gases before the cat rather than after the cat as in gen4s. The gen3 egr cooler is also smaller than gen4. When the egr cooler clogs enough it will code regardless of engine temp or startup status.

However the reliable egr valve is closed at startup, low speed driving and wide open throttle. It makes no sense that a closed egr valve would cause a cold start spark plug misfire. The misfire creates the rattle through a lose of engine rpm sync with the electric motors in the transaxle. The lose of sync causes a clutch like damper to slip until the plug starts firing shortly after startup.

Other plug, coil or injector misfires remain long after a cold start startup and code quickly.

Why does the plug misfire on cold start sometimes and not at other times? Why might the heater quit working after running with this problem for a period of time? Because a very small head gasket leak allows pressurized coolant into the cylinder when the engine is off. The spark plug is fouled taking a few seconds of engine operation to eliminate the moisture. All four cylinders now fire, sync is restored, the cylinder quickly heats up, the hg leak is sealed. During those initial seconds combustion gases and air fuel mix may get into the coolant and replaces the minor coolant loss. Some report "air" in the heater passages, reducing heat. Some don't, especially those reporting in spring and summer. After months of this, the head gasket gets worse and does not seal up when warm. Now enough coolant gets in to cause warm stop start shudders noticeable more often when running such as at traffic lights. By now the coolant reservoir is dropping slowly but not enough to overheat. Finally the cold start leak and rattle gets so bad it may cause an engine shutdown or white smoke from burned coolant. At this point the engine damage is often beyond a new head gasket repair, bent rods may cause poor compression or hydrolocked pistons may blow a hole in the block.

Once you have a hg leak nothing short of a new head gasket, at a minimum, will make it go away. What causes it? Look at the major reengineering the gen4 1.8L has compared to gen3. Yes, a whole new egr design. New pistons and rings eliminates oil burning. New cylinder coolant passages, geometry and an insulator. New tumble patterns into the cylinder. A new intake manifold.

If you have another shudder/rattle at cold start have a borescope inspection looking for a "steam" cleaned piston.

 

It shouldn't cause misfire diagnostic trouble codes, but it absolutely can cause rough running at start-up:




My own EGR valve was replaced under this campaign (about 5,000 miles after a head gasket replacement AND EGR valve/cooler cleaning). The electronic stepper motor on the original 2010-2013 EGR valve is not as reliable as later versions and it does sometimes fail ( Toyota Prius P0401 EGR Insufficient Flow: Solved | Torque News ). Toyota updated the original valve (25620-37110 to 25620-37120).

 

Unfortunately, I'm pretty sure the OP's issue is a head gasket failure. Been there done that.

I'm also the 'lab rat' and/or 'idiot' who used Bar's Leak HG-1 in my high mileage Prius v wagon. See this post :

Hi all, new Prius v owner | Page 12 | PriusChat

It has been going strong since Sept, 2021. However, as I say in my linked post, I'm still super skeptical...and don't think this will be a long term fix. But who knows...I'm going to keep driving it until 'something bad happens'...and then get rid of the car. Just not worth it to dump head gasket type money into it at this point.

BTW - I got the car for $8500 back in 2018 when nobody seemed to want those pesky hybrids. Amazing how the current market differs from back then! Hopefully the car will last a bit longer...until this crazy used car market gets back to some semblance of normality!

 

Toyota BS'ing up a storm...

 

Commenting on why Toyota used EGR we will never know. Although I suspect it is mostly for emissions like how most cars swapped to EGR.
OP did you try unplugging the electrical connector on the EGR valve yet? This is one of the low-hanging fruits to diagnose misfires on these cars. If the misfire goes away while the EGR is unplugged it means you still have uneven or inadequate EGR flow. When you unplug the valve it swaps to a map that runs without EGR flow as a safety precaution for actual EGR failures. We would assume that would mean limp mode (like other vehicles I have had) but on this vehicle, it does not go into limp mode.
Heater issue I can't comment on.