Rough Start Only On Warm Engine?

New intake manifold gaskets were used on reassembly.
@negusm I will be using your parts list to order new hoses in the near future. Thanks for your detailed input! Much appreciated.

 

Maybe a dumb question, but would a small hole in the muffler cause a P0171 code(running lean, vacuum leak)??

 

That is so far passed the AF sensors...I really doubt it.

 

So, got with my local guy (who I feel is trustworthy), and his final word on it is, low compression on no.5, aka, very likely a cracked head and therefore unsalvageable without replacing the engine. I'd like to think he's wrong, but at this rate, I'll spend more than the truck's worth trying to get to the root problem. Haven't done the injectors or regulator, and it doesn't seem like coolant's getting into the engine, so I'm not sure I agree with his assessment, but I know it's a real possibility. Not liking the idea of spending 30-40k on another vehicle, money's tight enough as it is. Still doing exactly what she was doing, runs fine from cold start, but as soon as you turn it off and restart, the whole engine's shaking from the misfire. Crap.

 

Possible, but I couldn't point at the 02 sensor for loss of compression (I don't think), which my local guy is saying means this truck needs a new truck. I'd like to disagree, since I'm not in a great position to buy another vehicle, but he has a point. I was just sure that if I had a cracked head, I'd be losing coolant into the engine, and that doesn't seem to be happening. So, back at square 1, scratching my head.

 

kdlaz, If you have low compression, you have a big issue. I'd more think the piston rings are worn out.

Did he do a wet and dry compression check? That may clue you in to what is happening:

If the compression results remain about the same: The low compression is likely due to wear in the valves or valve guides.
If the compression results improve significantly: The low compression is likely due to wear in the piston rings
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I don't know why he went straight to a "cracked head". Also, I don't know why he thinks a new head is as expensive as a whole engine.

So yeah, I'd chase the Compression issue first. Rent\borrow the tool and just do it yourself. Do a wet and dry test. If it's your rings, you are probably SOL. If it's the valves, a new head may fix it...or a valve job if anyone still does those.

It could be a cracked head...and it might not cause leaking into the coolant. It's possible. Again, I'd think a new head would be cheaper. The engine doesn't have to come out on these cars from what I can see.

 

Thanks. I don't know offhand how he tested the compression, but I agree, e new head would still be cheaper than a new engine (or a new truck). I'm considering having someone else look at it, second opinion might be good at this point. I do know another guy who might do the valve job, and it'd probably still be cheaper than a new engine/new vehicle. Not really in a position to support a 3rd car payment, we're already pushing it on the monthly budget.

 

Down the rabbit hole I go:
So, update if anyone is still following. Found the vacuum leak & it was one of the newly installed fuel injectors. Turned out to be a bad gasket. P0171 lean code code gone. So, thought I finally fixed the rough start when warm issue. NOT, it still starts rough/shakes when it is in a certain temp range. Not throwing any codes yet, but it’s only been a day.
I honestly don’t know what could be causing the intermittent rough starts now that most of the culprits have been replaced.
Stay tuned. My OCD will get the best of me soon!

 

A shake like that is sometimes a bad fuel injector.

 

Highly Doubtful. Pulled the originals thinking they were the problem. Replaced with MotorWest (Denso) fuel injectors, same problem. Then had the bad gasket on one fuel injector & MotorWest sent me 6 new fuel injectors & still having same issue.

 

Understood. I agree that with what you've done, they wouldn't be the issue either. Might be the wiring now.

 

So, my 17 year old son seems to think a supercharger will fix the problem…

 

That's a fast way to lunch a 20 year old transmission.

 

Any update to this? I’m having the same issue with the rough start up on a warm engine.

 

As mention, 20yo injector are probably toast or clogged. Motorwest can clean and flow test them.

 

Well, not really any update. But, it still happens once in a while and it is not as bad or frequent as before. So, some of my fixes must have helped a little.
I ran this by a Toyota Master Mechanic & he said to check/replace plugs/wires as they can cause some crazy issues like I’m having. I will probably do that in the near future.

 

Check all grounds. Can't remember if this was mentioned in this thread.

 

Still haven’t nailed down the rough start when warm. Fifteen minutes seems to be the magic timeframe when it occurs. This is after its already warm.
Recently replaced the PCV valve, and had new plugs/wires put in. Old plugs looked okay & were correct plugs. No help though.
Open to suggestions. Someone has had to have similar experience.
Thanks!

 

Just an update to this thread. I recently bought a 100 series Land Cruiser & found some interesting information about vapor lock on some. It’s very similar issues, but they have a check valve in the fuel rail. I don’t think my 2002 4Runner has one in the rail, only a fuel regulator which I replaced already.
BUT, there is a check valve on the fuel pump and when it starts to go bad you get the exact symptoms that I have. Just wanted to pass this information on as I’ve not heard it mentioned on this forum. This has to be my issue as I think I’ve replaced everything else. I am going to keep running it as is for a while & if it gets much worse I’ll replace the fuel pump.

Happy Thanksgiving!

 

Finally, problem solved! This guy Ivan is amazing and lives nearby and I never knew about him. Yes, this is my boys 3rd gen and the vehicle that started this thread. It runs great now that I’m done shooting the “Toyota Parts Cannon.” Just wanted to follow up with the fix so others could benefit. Not many mechanics diagnose like this these days.

https://youtu.be/uCriDDdw-V4?si=wEaDtsLNmn13dmSL

 

What a great diagnosis of a weak fuel pump. It's right on the edge of failing and giving misleading symptoms. You're right, few shops will actually diagnose like this anymore. Thanks for sharing.

 

My takeaway from reading a lot of these threads and my own experience is that among the parts in this chain, the fuel pump has both a higher probability of failure and a lower probability of being replaced in the parts cannon phase than almost anything else. Most likely that's because of what a pain it is to access.

 

It seems to me that one of the best thing a car manufacturer could have done was to measure fuel pressure and report via the OBD2. Maybe more modern cars do.

Faulty Fuel pumps cause so many issues but never seem to throw codes.

 

Here is Part #2 and the final repair with detailed diagnostic of my bad fuel pump. In the long run I replaced a few parts; fuel regulator, fuel injectors, plugs/wires, MAF sensor, A/F sensors, PCV valve and finally the culprit the dreaded fuel pump. They were all due with the exception of the fuel regulator, so I don’t feel as I’ve wasted too much time/money. But, boy does she run great now!
Go Steelers!

https://youtu.be/HiK8O7u8K4g?si=oslyTqVF4rPxCysP

 

Very thorough. Thanks.

 

If it wasn't done before, you may have wanted to replace all the tank gaskets, the sender and that pesky valve that cracks after 15 years. Then the tank doesn't have to drop again.