2.7l Will not start when hot - ANY ideas?

Hey guys.

So has anyone looked into the possibility of this being vapor lock? It sure seems to act like it. My truck starts immediately after shutdown, then after 20-45 mins won't start, then after waiting until cooler it will restart. Although it's not common on fuel injected engines, it still can happen.

Has anyone replaced his fuel pump? I wonder if aging fuel pumps are unable to make proper pressure when hot. Lower fuel pressure = more fuel vaporization. Add to that a recirculating fuel system (that warms the fuel), winter gas (the added ethanol lowers the boiling point), and an exposed fuel rail on top of a hot engine could cause vapor lock.

And maybe the cooler IAT temp data (either real or simulated) is compensating by opening the injectors longer. This may be allowing the vapors to be expelled through the injectors and the engine starting again. I do know that if I just open my hood a few (3-5) minutes, it usually restarts.

I'm wondering if replacing the fuel pump might solve this issue. That and insulating or heat sinking the fuel rail. Since I moved my IAT sensor I haven't had an issue but it hasn't been as hot lately. The one warm day it took a little longer but still started. Thoughts?

 

Just bought A 1996 2.4 Tacoma, and I have the same problem. I noticed that mine has an exhaust leak at the manifold. That has got to be putting a lot of unwanted heat in the engine compartment. I noticed the leak when cleaning the intake w/ sea foam. (it's also loud)
It sounds like the switch is the best way to go, and just live with it. You guys sure did save me a lot of money. Has anybody tried the canister?

 

The exhaust leak at the manifold is causing you to run rich because the O2 sensor sees a lean condition. This condition must be fixed first before you can do anything else. A rich running engine produces CO which can be dangerous leaking into the engine compartment and possibly into the cab.

 

The exhaust leak isn't going to be significant enough to overheat the engine compartment or cause the truck to not start. Mine also has an exhaust leak (a crack between cyls 2 & 3). Before you try the switch/resistor combo, try cleaning the MAF sensor by removing the sensor and spraying it clean with MAF specific cleaner. I like the CRC stuff in the gray can. Let us know if that makes any difference.

The reason I suggest this is the last time I had the truck not start, I relocated the IAT sensor to inside the fender then a couple days later cleaned the MAF sensor. I'd like to know if perhaps the MAF is the only problem. Like I said, I haven't had the problem since but then again, it hasn't been too hot yet. This week is supposed to be hot (I'm in So. Cal.), so I'll let you know if I have any problems.

As far as the canister, I tested mine and it pretty much checked out. I don't see how it could be related. I also find it interesting Toyota changed the MAF sensor (I believe) in 97 or 98. Perhaps they discovered the problem and made the change for that reason or it was just cheaper. With the newer MAF, the IAT is integrated into it.

 

On any OBDII vehicle, the O2 sensor doesn't even come into play until it heats up and that's not until it runs for several minutes. It certainly isn't involved in starting the vehicle.

And while true an exhaust leak could cause the vehicle to run rich, in most cases it won't be significant enough to make a difference. Case in point: my truck has a cracked exhaust manifold too and it passed a CA smog check with flying colors.

ALL engines (rich, lean, or running at the perfect stoichiometric ratio of 14.7:1) produce CO. It's a byproduct of combustion. I think you're confusing CO with HC (hydrocarbons). A rich running engine will have a high HC reading. With my last truck (a carbureted 1987 Toyota), I needed to lean the mixture before smog tests otherwise I'd fail based on high HC. I'd have left it lean except it was so much more drivable when enrichened just a bit.

 

Stoichometric is the ideal mixture at 14.7:1. No I am not confusing HC with CO. When HC (unburnt fuel) is high you have incomplete combustion in the combustion chamber which also produces CO or Carbon Monoxide which is what kills. Passing the smog test is not a measure of engine combustion when you have exhaust leaks. A cracked manifold or manifold exhaust leak tends to pull air into the exhaust between the closing of the exhaust valve and the reopening of the exhaust valve which at the tailpipe would indicate a lean running engine. A 4 cyl engine tends to amplify the problem. Incidentally, CO is minimized at 14.7:1 ratio. What is coming out the tail pipe when the mixture is at 14.7:1 is mostly CO2 and H2O. Yes a rich running engine does tend to run better and produce more power however keep in mind what is being produced in the combustion chamber. Calif inspection as in most states which still have an inspection program are looking at what comes out the tailpipe not how the engine is running. The OBDII system is actually a better inspection station than what the state is doing. The state continues their inspection as a tax on you the consumer.

 

This!... I'd bet it's a sensor somewhere. The starting process depends heavily on information received by the computer. That information comes from a whole bunch of sensors. My guess is look at the fuel system...

 

Ok, so how does any of this cause the truck to not start when hot on a hot day?

 

Been there, done that: 185-190F

 

Computers have a learn feature that can remember a rich running condition and put too much fuel into the combustion chamber when the vehicle is warm and cause a rich condition when it should be lean. This could be cleared by holding the accelerator pedal all the way to the floor and crank engine. I see a lot of reference to the IAT as being the possible culprit which very well could be as a heat soak problem. You should be able to tell with an ODBII scanner hooked up when the problem occurs. I can only offer suggestions based upon my previous experiences as I no longer work in the trade. I have not experienced the problem on my Tacoma as I have a 2012. I just offer suggestions when I think I can help which you and others can take and use as you see fit. I cannot compel you to do anything. I have over 50 years in the Auto Tech trade and taught Automotive for 20 of those years.

 

Interesting - it's getting hot out, so I'll put this theory to the test. Thanks, GWC!

 

Not sure that I would agree with this. The smog stations have a way to detect HC and CO. We only have an 02 sensor which only tells if we are rich or lean, and no way to monitor CO. Plus their equipment is calibrated on a regular basis with a calibration gas much like the gas used by 5 gas analyzers. The thing that is crazy is that diesel truck(F350) not semis don't have to be smogged. Those thing spew out all kinds of NOx and Soot which is very bad for peoples health

 

Your modern automobile has 2 O2 sensors and in some cases 4 O2 sensors. The first O2 sensor monitors what the engine produces and the 2nd O2 sensor monitors the efficiency of the Cat. The only thing the inspection station monitors is what comes out the tailpipe. A good inspection station may detect a small leak in the exhaust or may not. The rise in O2 coming out the tailpipe is a good indicator but you have to remember what the Inspection station is looking at.

Most states have done away with the inspection simply because the computer in the car does a better job of monitoring. The check engine light is only used for smog related problems! If you were to hook up an OBDII scanner and look at the parameters your engine is operating under you can tell more about what is going on than any inspection station.

Diesels produce very little NOx compared to a gasoline engine as a diesel is a compression ignited engine which has more complete combustion. Does not have a problem with quench areas as a gasoline engine. The soot produced by the diesel is a problem but can and is eliminated in a correctly running diesel. Diesels are inspected where inspection is by doing an opacity test which purpose is to determine the amount of soot being produced.

Incidentally, I taught the inspection station operators when they first started the inspection in Florida. I believe the public would be better served with a safety inspection rather than an emission inspection. With a safety inspection we could at least get people to repair brakes and replace lights. But, then again what does all this have to do with the hot start problem?

 

^

 

I'm from Ohio. What's an inspection!....

 

...and so our good thread about a no start problem turns into one about tailpipe emissions.

 

Modern cars have 2 O2 sensors per bank(exhaust manifold). I assume that you are aware how narrow band 02 sensors work? They are digital devices and only indicate whether you are in a lean or rich condition. The smog station uses wide band technology which measures a number of parameters(HC, CO and A/F ratio. So my point is that the smog check station is giving much more information than the narrow band O2 found on your vehicle.

This is simply false; there are hundreds of codes that can be thrown some of which are emissions related but others are not. For example the B, C and U codes have nothing to do with emissions. OBD-II Diagnostic Trouble Codes are 4-digit, preceded by a letter: P for engine and transmission (powertrain), B for body, C for chassis, and U for network.(https://en.wikipedia.org/wiki/On-board_diagnostics)

Where do you get this stuff from? The following link is not from a blog, but from an actual scientific paper. Check out Table 1 in the paper linked below:

http://www.researchgate.net/publica...ed_by_uncertainty/file/79e4150b2581717d26.pdf

Sorry Rick, you are correct about this not being a good place to debate emissions. Any further discussion on this subject should be in another thread.

 

Has anyone found a fix for this hot start issue?

 

NICE FORUM! good thread.

My 1996 2.7 4runner with 160k miles just started doing this .. open throttle just a little to start. I will let you know after I try some things in the thread.

 

Same problem with my old prelude, the fuel relay contacts were corroded.
Had the exactly symptoms as the op