2.7l Will not start when hot - ANY ideas?

I am willing to try anything to help the community with this problem, but will not be able to test until summer time when the problem occurs. I am still trying to understand how artificially lowering the IAT solves the problem. I cannot understand how this is related to the CPS(crank pos sensor), but I could be missing something.

Toyota dealers must have seen many cases of this problem. Are there any Toyota mechanics on this forum? If so, is there a way to make them aware of this thread?

 

Perhaps someone can post the page for me but per the FSM page EG-196 (for my 1996 FSM), if the computer is unable to provide fuel and spark calculations from the sensors, it will provide default values. I think that's what's happening when you unplug the IAT sensor. Missing that data, the ECM reverts to what's programmed as its fixed backup data. It sets timing at a fixed 5 BTDC and fuel based on the TPS. It goes on to say it will illuminate the MIL but not throw a code when in backup mode.

When you unplug the sensor, I think the fuel cut fail-safe of the CPS is being overridden by the missing IAT sensor and instead of cutting the fuel for a failed CPS (like its designed to), it supplies default data for everything (fuel and spark) but only when you unplug the IAT sensor. Otherwise it thinks, "bad CPS = cut the fuel".

I'm sure there are some Toyota techs who could chime in on this issue as it seems both very common and elusive (usually that means a trip to the dealer as a last ditch effort when people are desperate).

We've been having some unusually warm weather here and I've noticed it more frequently since (I've only had the truck about 1 1/2 months). It's definitely a heat sensitive issue and in the engine compartment. Tonight, however, I'm going to unplug my CPS and try to crank the engine to see if I get a code with the sensor disconnected. I'm also going to OHM it per the FSM readings and see how it does. Another good test might be to spray some of that air duster stuff on the sensor to cool it when it happens to see if that fixes it (not as a liquid, just air).

 

Interesting, but the way that I have mine hooked up it should never go into backup(limp) mode. The switch that I showed above merely adds 470 Ohms to the circuit, which is still a very legal value to the ECU. Also, my engine check light doe NOT come on.

Spraying air duster on the sensor should have a similar effect as my resistor mod. The CPS is absolutely essential for the engine to run on most EFI vehicles, but since our trucks(1996) still have a distributor the computer could get rough crank angles from the distributor. Not sure if this is true, but it would be possible. I would be really interested to see what you find.

 

page E-196

 

Sounds like someone already replaced their engine without fixing the issue. Who wants to volunteer to switch out their wiring harness?

 

just putting this info out there. if you 2.7 guys want your motors to last get your valves adjusted every 100K I keep getting these motors coming in not starting and missing hot due to burnt valves from being held open..


I don't work in a toy dealership, but this is becoming very common..

 

It certainly doesn't appear to be a harness. It's something that under heat keeps the engine from starting. Mine has never stalled while driving. Although a faulty harness could cause a no start problem, it would be very rare to have so many of them.

 

Not sure if you're referring to me, if so the engine was rebuilt before this problem showed itself.

 

didnt bother reading all the posts but if it wasnt mentioned i think its leaky fuel injectors or the injector o-rings

 

Tried it.

 

Fuel injectors were cleaned and tested and installed with new seals, no difference. Also had the MAF sensor checked.

 

New thing happened today. Still have the hard start issue when hot. Today I parked on an incline. Turned truck off for about 10 minutes or so. When I went to leave, it was hard to start, but once it started, it stumbled and died. Started it again, same thing, it died. Go for a third time, it finally cranks, starts to die, and I quickly push the gas pedal. Engine stays on.

Seems like mine is not getting fuel when trying to start. Fuel pump maybe? Doesn't make sense to me as to why it would start cold fine, then not when warm if it was the fuel pump though.

 

Years ago when I worked in a Toyota dealership we would get cars with a similar problem. In most cases it was cured by going to the correct spark plug. Toyota's do not tolerate many of the replacement plugs.

 

What plugs and gap do you recommend?

 

speedojosh, It has been years since I worked in a Toyota dealership but, I never used anything but NGK plugs in my own Toyota's and gap to factory specs. When I was running an oval track car I would gap a little narrower because it would prevent breakdown at higher RPM. Coil saturation time decreases as RPM goes up. BTW, I retired in 2002 from teaching Automotive so I am totally away from it today other than when I work on my own stuff infrequently.

 

I just replaced the plugs in my truck (2.4L). I removed Denso Platinum gapped at .040 (they ran ok). I installed NGK BKR5EYA (aka stock #2087) gapped at .032 and they're great. I agree with the NGK only comment for Toyotas although Densos are good too. I've seen people use Autolite, etc. but I'd rather not.

 

So, I've had the failure to start when hot issue a couple times recently during some abnormally hot January weather. Sure enough, popped the hood and let it sit for a couple minutes and it fired right up. Same when I disconnected the IAT sensor; it started right up but triggered the MIL.

I also did a little testing with my ODB2 scanner on a cool evening. After shutting the engine down, I watched the temps read on both the IAT and coolant. While running, the coolant stayed about 185F and the IAT about 57F (ambient was around 60F outside). After shutdown, however, the coolant temp peaked at a mere 190F while the IAT kept climbing and peaked about 30 minutes later at 102F (people mention "heat soak"). This seemed like a large increase to me but it makes sense as it's reading the heat from a hot engine at the top of an enclosed engine compartment.

I also found a Toyota published overview of temp sensors here: http://www.autoshop101.com/forms/h32.pdf On page 2 it says one strategy the ECM uses to determine a cold start is when the IAT and the coolant sensor are within 15F of each other. So this got me wondering if on a hot day, perhaps the IAT is reading a temp very close to the coolant temperature (falling within 15F of each other) and it's flooding by assuming a cold start when in fact it's hot. Example: coolant temp drops to 150F while IAT rises to 140F.

So I'm now contemplating relocating the IAT to the air intake horn OUTSIDE the engine compartment (in the fender instead) to keep it farther from the heat produced by the engine. It should still read the intake air accurately but hopefully won't cause the hot no-start condition. At this point, the resistor bypass switch is also a tempting option but I'd prefer to find the real cause of the problem first.

I also replaced the ignition condenser in the distributor after reading they can sometimes short when hot. No luck, but anyway it was a cheap thing to eliminate. While I had the distributor out, I checked the pickup coil gap (it was fine) and ohmed the pickup and ignition coils (both were fine).

 

samwise, I'm starting to look more into this resistor modification. How did you come up with the 470 Ohm value? I'd be interested to see how much that changes the value read at the ECM if you were to switch it on then off with an OBD2 scanner attached like in the experiment I attempted above. It looks like it could read about 35F lower than actual with the resistor in the circuit, depending on the ambient temp range. I agree that it should appear as a very legit value to the ECM (140F=600 Ohms and 104F=1100 Ohms per the FSM).

I'd also be interested in seeing the temps of both the coolant and IAT when the no start occurs if there's anyone who can do this (perhaps with an always on/connected ScanGuage?).

 

I used to have a potentiometer(POT) in place of the the resistor and fiddled with it for about a year. After a while I got a pretty good idea of how much resistance I had to add to get it to start. I then took a measurement of the POT and replaced it with the switch and resistor. As for plugs, I did NOT use OEM plugs. I might give this a try. Has any replaced the ignition coil? Warn plugs have increased gaps, and increased gaps have more resistance. So a low high side voltage due to heat could cause a weak spark, just a theory. If I had access to an ignition scope, I would definitely check the hot and cold start voltage. I looked at a 98 and noticed that it no longer has the IAT sensor. They either moved it or combined it with the MAF sensor.

 

I have tried the coil, along with several different plugs. At one time I actually carried extra plugs in the truck so that when it fouled the ones in the engine I had a back up......it was a headache. Nothing seems to fix this ongoing issue.