All about injectors

[devnull]

I've got a 99 Yukon.. I was told the engine was replaced 40k miles ago because the original owner ran it hot, but I don't know if that means a completely new crate engine or what. So how would I know if it has poppet injectors? Is there a part number or something I can look for around the intake or the connector?

Secondarily, my fuel economy is bad lately.. been getting very mixed results on the highway, sometimes an entire highway tank yields 14mpg, other times 15.5, and i've never seen it go above 16, driving mostly 75 to 80 with the cruise control. Assuming I first replace the usual stuff like the rotor, cap, plugs, etc (fuel/air filter have already been replaced recently), how would I know whether injectors are my problem in the absence of misfires/codes/etc? Are there any "handwriting on the wall" type signs I can look for with standard tools and the HPtuners software?

 

[Phone Man]

Getting 14 to 15.5 at 75 to 80 mph? That's pretty good.

 

[Gzes]

Yea thats great i bary get 14 on good days. Most likely they just slapped another engine in or rebuilt it i highly doubt they stuck a new.crate emgine in

 

[19Yukon97]

Thats good mileage at the speed. Mine gets 19.2-19.5 at 68 with the cruise. When i set the cruise at 75 it goes way down to 14.5. You have to remember these trucks are only advertised at 16 mpg.

 

[devnull]

Thanks for the replies.. I'll definitely try going closer to 68 and see what happens. I've read that the stock setup seems to be most efficient right at 2k RPM on the highway, which falls into the 68 to 69 mph category if I remember correctly. Of course the ultimate would be if I could make it do 19.5 mpg at 75mph by way of "efficiency upgrades" like an entirely new exhaust and a little tuning. The reason I'm suspect of the injectors is that I feel occasional "power bumps" on the highway, sometimes even without the cruise control, leading me to believe I should tackle everything ignotion-related, coil-to-plug, then start looking at injectors if that isn't fruitful.

Closely related, what's the story on the distributor gear/shaft for these engines? I seem to recall reading that wear on the gear becomes an issue after a while. Car has about 100k miles, but no telling how many miles on both sides of the gear because of the engine replacement I mentioned previously. Thanks.

 

[SunlitComet]

inspect you ignition system for fualts. if it is random drop outs. old injector design just has thin tubes. new design has thicker tubing with wires along side with abrasion tubin around it and injectors at the outer end.

new:



old:

---------- Post added at 01:56 PM ---------- Previous post was at 01:55 PM ----------

the dizzy gear is replaceable lone if worn.

 

[used97]

That's a funny thing that you get power bumps on the freeway mine did too until I just replaced the fuel pump. I tried everything from replacing all the wires, plugs, and etc. even changed the fuel filter thinking that might be the problem. Don't change the injectors unless absolutely a necessity. If you remove the intake manifold you might open up a can of worms. Just my .02

 

[SunlitComet]

not bumps those are dropouts. and what can of worms are you talking about?

 

[devnull]

That's a funny thing that you get power bumps on the freeway mine did too until I just replaced the fuel pump. I tried everything from replacing all the wires, plugs, and etc. even changed the fuel filter thinking that might be the problem. Don't change the injectors unless absolutely a necessity. If you remove the intake manifold you might open up a can of worms. Just my .02

Interesting, i'll keep that in mind about the fuel pump if all else fails. Being 13 years old, I can easily see that it might be time. I've only felt the 'drop out' on the cruise control a handful of times, and on manual throttle once. It was so minor that it could've been anything really.

Also regarding above in the thread, "these trucks are only advertised at 16 mpg" - mine says 18 on the original paper in the window (there's probably a better name for that), and it goes on to say that it was based on testing that yielded between 15 and 21 MPG. I sure would like to be in that 21 MPG category Maybe some day with upgrades.

Thanks for the pics of the injectors, I've seen both kinds before. What I was after was a way to determine whether I had the old style or something else, simply by looking at the outside of the plug body. Do they have part numbers stamped on like some of the others?

 

[SunlitComet]

you should be able to remove the air intake duct hold the throttle open and peak down to see a line possibly. but between the electric pins of each injector do you see a small hole thru the plastic body or a "t" like opening in the body?