96 Yukon hard to start when warm, backfires, smoke

gmc
yukon

#1

My fellow Car Talk fans,

I have a question for you that has stumped the finest minds of Victoria BC Canada. From what I’ve seen online there are other folks who’ve had it and not found a good answer. My truck is my livelihood. I build tiny homes. Thank you.

My 96 GMC Yukon 5.7L is a great beast but has developed a troubling tendency: it struggles to start when it’s warmer out and after driving, doing a short stop, and starting up again. Every so often, it’s so bad it makes a loud backfire with a puff of awful smelling smoke coming from under the hood toward the left.

I’ve done everything the Haynes manual says (air filter, fuel lines, distributor contacts etc) and shown my normally ace mechanic other ideas out there online (timing belt etc), to no avail. I want to understand what is going on, to make the truck safe and reliable and to learn more about how vehicles work. Thank you & Have a great day,

Forest
Victoria BC Canada


#2

The fuel pressure regulators on these things are notorious for leaking. You’ll find it under the upper intake. Remove throttle body and check for wetness in upper intake. If it’s wet in there then the regulator’s leaking.


#3

Thanks very much Pete.
As far as I know the throttle body injector only goes to 1995. Mine is a 96, so on the MPFI system. Does that make any difference?
Have a good day
Forest


#4

An excellent post by Pete.

An additional possibility if this turns out to not be the regulator is the temp sensor. It its curve isn’t what it should be, the ECU may not always know the correct temperature of the engine. In Victoria that can have a very noticeable impact.

There are other possibilities, but Pete’s post and the temp sensor are good starting points. If you’ve had a CEL, the codes from that might help too, but since the ECU doesn’t store codes during start up that might not offer any help.


#5

Sounds like it’s flooding with fuel. I would replace the fuel injection spider.


#6

Car Talk Lackey
Engine backfire is caused by an imbalance in the air-to-fuel ratio of the vehicle. Backfires occur in one of two places. A backfire in the intake manifold is caused by a ratio that is too lean (not enough fuel). A backfire out of the exhaust system is caused by a ratio that is too rich (too much fuel).


#7

Is this a fuel injected or carburetor vehicle? assuming fuel injected but just want to know for sure.


#8

Fuel injected . . . “spider”


#9

@db4690 spider was in a comment not by the op, just double checking, Who knows what goes on in canadia


#10

All good ideas above. My first guess – if this problem only happens when the engine is warm, and otherwise the truck runs fine as usual – is a failing crank position sensor. These parts tend to fail only when hot (at first), and when they do it can cause a misfire. Everything else works, the engine turns, fuel is injected, but no spark at the spark plug. That allows unburned gas to go clean through and out the engine, and unburned gasoline could easily backfire or smoke on the way out the hot exhaust system. Best of luck there Forest. A very good name for a resident of Victoria Canada by the way … :wink:


#11

Yes, fuel injected, thanks. 96 was the first year of multi port fuel injection for the Yukon. Before it was throttle body injection. What does that affect?


#12

You guys are the best btw


#13

“The best” wouldn’t ask if a 1996 Yukon has a carburetor. Listen to Pete, he is a GM tech.


#14

You still have a throttle body.


#15

The truck does indeed has a throttle body

However, it does not have TBI . . . please trust me on this. We have/had plenty of GM trucks of this vintage in our fleet

It has a newer version of the “spider” fuel injection system. Starting 1996, it was CSFI, which still had a central injector, with poppet nozzles the end of those plastic lines. A few years later it was MPFI, which is yet another variation of the same system. This time, a central fuel injection system, with mini-injectors at the end of the plastic lines. It’s possible OP’s car has already been retrofitted to MPFI . . . it’s a drop-in, BTW

What does this affect . . . there’s still a bunch of hard plastic fuel injection lines under that plastic manifold. Lots of stuff to break and/or leak. And the fuel pressure regulator is also under there. The good news is it’s pretty easy to diagnose


#16

Wouldn’t a leaking fuel pressure regulator show up in a fuel pressure test? If so, that’d be easier to do than removing the throttle body to check for wet fuel underneath.

If the fpr can leak into the intake manifold without a fuel pressure test showing it happening, then this symptom is consistent. Fuel leaking into the intake manifold while the engine is turned off, then trying to start the hot engine, it will act flooded, very stubborn to start, and cough and sputter when it does catch.

I had a similar problem with my carb’ed truck this past summer. The carb’s power valve was – unknown to me – leaking fuel into the intake manifold. Similar symptom, the truck would usually start normally cold, but very stubborn starts when the engine was hot, especially if I had been driving it, turned it off for 15 minutes or so, then tried to start it. I discovered by trial and error I could get it to start much easier in this situation by pressing the accel all the way to the floor and holding it there during cranking. That was the clue I needed, overly rich for some reason. Removing the carb from the intake manifold showed it to be dripping gas.