Rough Start Only On Warm Engine?

Starts great when cold & immediately after running it for a while. BUT, when it sits between 15-60 minutes, it’s hard to start & sputters when it doe’s finally start???
I’m thinking fuel pump is starting to go? Any thoughts?
FYI: Baselined this rig for my son when we bought it. Fuel filter is new, tried fuel injector cleaner, air filter is new.

 

No codes?

My experience with bad fuel pumps is that they make it hard to start when cold. But my experience is limited. For one car, I cycled the key on and off a bunch of times before turning the key to start to get the fuel pump to pressurize and then I could start it. You might try that to see if it makes a difference.

These are the worst because until the thing actually won't start, it's hard to know.

 

Off the top of my head,

Have you had a moment to inspect the spark plugs? I have seen an instance of where after shutdown, An injector could slowly leak creating a possible hard start condition.

 

I get the evap emissions code off and on. Also get the running lean code once in a blue moon.
I need to pull the plugs & take a look. Guess it could be injectors also.

 

I agree. Pulling plugs is simple and free. It can tell you a bunch.

 

Plugs, wires, coils. I've got the same trouble. Once it warms up, it starts throwing the 306 code. Misfire on cylinder 6. I've replace the plug and wire, so far. Going to get the coil replace tomorrow. We'll see what happens.

 

PhantomTweak, swap coils first. Free and easy and if the misfire moves, you know it was a coil.

 

So, a little update. Ran the vehicle past my favorite local “Old School” garage and my favorite mechanics that I actually trust.
I was able to duplicate the hard start for him. He is thinking possibly the “Fuel Regulator.” But, said bring it in next week & he will look further. My knowledge is limited in this area, but after a little online research, he may be onto something!

 

Well, bought a fuel regulator from McGeorge Toyota then 6 fuel injectors from MotorWest just in case. My mechanic had to take off intake to change the fuel regulator. So, ended up changing injectors/fuel regulator and D/S valve cover gasket while it was all off. Bigger job than expected, but had to be done sooner or later. P/S valve cover had been done recently, so only D/S needed done.
Started fine & ran great, but had a rough idle. Smelled a little gas too. Thought the ECU might take time to reset the idle. Then warmed it up the next day & saw gas dripping underneath! Hopefully just a fuel line needs tightened up. Too cold out to work on the driveway. Will take it back to the shop Tuesday.

 

Update: Leaking fuel injector! Last one against the firewall. So, will have to pull it and replace the rubber O-ring. Luckily, MotorWest gave me extra’s when I bought the fuel injectors. Hopefully, it gets done today. I’m curious to see how it runs with new injectors & if mileage improves.

 

Well, back to the drawing board! Fuel injector fixed and the vehicle is doing the exact same thing it did before the new injectors and new fuel regulator. Now it is throwing a P0171(Too Lean, Bank 1) code that I would rarely get before. Hard to start when warm, not hot or cold. Not sure of my next move, but at least my son has new injectors, fuel reg and valve cover gasket.
I’ve cleaned the MAF sensor in the past and may buy a new one.
Open for suggestions. Little deflated, as I thought it had to be one of the two fixes.

 

Has a compression and leak down test been performed at this point? I would suggest that after the amount of items already replaced to get an idea of what the general engine condition is.

Please update when available!

Good luck.

 

Same here. I'm trying to avoid the "do everything until something pans out" approach. So far, replaced lower and upper intake boots (upper was separating), and coil. Probably going to go at the fuel injectors next. Haven't put the diag on it, so I don't know if it's coding or not. Love this vehicle, and have put brazillions of miles on it, but wouldn't mind putting another brazillion or two, assuming I can nail down the repair lottery. Can't really afford that approach. There are a couple really good Youtube videos out there, but I'm not ready to tear the engine apart just yet. Hoping to figure it out quickly (and as cheaply as possible).

 

I understand and agree 1000% with what you say and want out of your vehicle.
With that being said starting off on a good diagnosis takes good input on what issues you encounter when you have a problem with your 4Runner.
I am sure there are multiple videos on youTube showing the diagnostic techniques that can be used in narrowing down the issue and ultimately the culprit part needing to be replaced to correct your issue.

good luck and check back with updates!

 

Will do. With the work so far (upper/lower intake boot, coil), I'm in for around $450, with no real improvement. Fingers crossed we get it nailed down quickly. She's my main ride.

 

I understand what you are dealing with and only hope this can be nailed down shortly in regards $$$$$ outlay as this is a major issue affecting you. Will keep and eye on on updates.

 

this may be dumb, but what about the O2 sensors? or even muffler/cat not putting enough back pressure on the system? It would at least be the Occam's razor solution

 

Update: So, I bought a new OEM MAF sensor & when I pulled it, they didn’t match! Prior owner must have put in a cheap MAF sensor(pic below). So, I’m onto something, right?
Put in new MAF and ram if for a while, kept throwing PO171 & still starts rough when warm! Now it is also throwing a P0300(random misfire) code.
Now, let’s recap: Problem is hard starts when warm only, not hot or cold. Replaced fuel regulator & injectors to no avail. Now replaced MAF with negative results. Current codes P0300/P0171.
Will have to sleep on this one.

Happy New Year!

 

P0171 is a lean condition air fuel problem.

The typical culprits seem to include the o2 sensors. Pull them and see.

Remember 2001 and 2002 4runners do NOT have o2 sensors. They have AF sensors.

You MUST check your vin at the dealer to get the right sensors. I assume you did this for your maf. If not, have them run your vin for your maf and verify you got the right maf too.

If a prior owner used a cheap maf, they probably used cheap o2 sensors that were not AF sensors.

 

Checked my STFT at idle and at under load and it appears I definitely have a vacuum leak somewhere. STFT was 15% at idle and decreased under load which indicates vacuum leak. So, the PO171 code pops right back up after reset but drives fine. Vacuum hoses are original and it’s probable time to replace them. I’ve seen some silicone hoses used by some or just go to the local auto parts store and get some standard rubber hoses? Anyone have a suggestion here for the vacuum hose replacement??

The new(correct) MAF appears to be working and I assume it may take a few cycles for it to make adjustments. Rough starts are not as rough and seem to be fewer. Hopefully I’m on the right track.

 

Buy the Toyota molded hoses and goto Napa for the non molded hoses. Get METRIC hoses and not "close enough" sae. My Napa was happy to order the correct size.

Here are the part numbers for a 2002: That is a thread I made refreshing my 4runner.

 

A link to that whole thread. My engine once looked as bad as yours.

 

Let’s not get carried away, I don’t think it looked that bad.
I found your pics on another thread, but not all the part numbers. Thanks!

 

Have you checked your pressure relief valve. You might be vapor locking. Use ethanol-free gas if you can find it.

 

Do you mean the PCV valve? If so, I have not pulled it off to inspect as it looks to be new.
If not, I have no clue what you’re referring to.

 

Pressure relief valve is at the end of one of your fuel rails. It is not the pcv.

 

I think it's also referred to as fuel pressure regulator

 

Fuel Regulator has been replaced already. A/F sensors getting pulled and most likely replaced tomorrow.

 

Replaced both A/F sensor’s yesterday outside in my driveway with 30 degree temps! Sons 4runner, I must like him…Pulled the downstream one and it was a a Denso. Looked a little worn, so it got replaced with OEM & I’ll keep it for a spare! Upstream sensor was NOT Denso, unknown brand & was a little shorter than the OEM replacement.
Drove a decent amount the last two days and still throwing P0171 code, BUT the rough starts seem to correct themselves vs stalling out now. I can live with the lean code until I find my vacuum leak. STFT is 15-20% at idle and goes down under load. So, really only an issue at idle. I’ll find the vacuum leak at some point.
I cannot say for sure what has fixed my rough starts. However, I think replacing the aftermarket MAF and A/F sensors with OEM parts maybe did the trick.
Appreciate all the input from everyone!
If anything changes, I will post updates.

 

Get all your hoses together, along with intake manifold gaskets and pull the intake. Then go through replacing all the hoses and the intake manifold gaskets and you will solve the issue.

Also, you aren't supposed to reuse the intake manifold gaskets. I see you already had the intake off, and if you didn't get new gaskets, that may be the source of your issue.