Stuck fuel injector?

I am experiencing a round spray mark on my rear window, directly above one of my trumpets. I am thinking that this is due to an injector being stuck in the open position. My engine is acting a bit odd on startup, and doesn't idle
as well as it should (flooding one cylinder?)

Will the existing rail pressure fill the cylinder with fuel upon shutdown of the engine, or does the injector have to "pop" in order to allow fuel through it? I'm no injector expert, that's for sure. What experience I have with them has been limited to marine diesels until quite recently. I don't want to risk "washing" the cylinder, or possibly damaging valves, piston, etc. Raw gas doesn't compress too well! /ubbthreads/images/graemlins/shocked.gif
I haven't pulled the spark plug yet to check for raw fuel, but that's the first thing on my list.
Anybody have any thoughts or suggestions?
Thanks in advance!

Hi Bill

An injector will not normally stick open (IMHO) but may be held partially open if a foreign object (grit, swarf etc etc) gets into the fuel rail and then into the body of the injector. If this is the case the injector will flow some fuel all the time the HP pump is running. A reverse flush can resolve this problem in most instances.

If the injectors are clipped to the fuel rail, you could remove the rails and with the injectors in free air turn the ignition on. They should not flow any fuel or even drip when pressurised.

Also - if you have a fuel pressure guage, monitor the rail pressure on switch off. providing your HP pump has a non return valve, the rail should hold its working pressure for some time after switching off from idle.

Finally, the Motec ECU will allow you to test the injector flow rates, one at a time using the inbuilt software diagnostics, you could then 'test fire' injectors in turn over a suitable collecting container for the fuel.

Let us know what you find.. /ubbthreads/images/graemlins/smile.gif

Looking again - as it is only on one cylinder - could it possibly be a sticky inlet valve that is allowing the fuel to be blown back up on compression?? /ubbthreads/images/graemlins/confused.gif
Hi Paul,

I appreciate your learned input!

The guys who tuned the engine on the chassis dyno, told me to expect some "residual stuff" from my hoses(S.S.&rubber, not S.S.&teflon, as they recommended). I blew them out with air and ran a bunch of fuel through them before and after terminating the ends, but apparently that wasn't enough. As you know, it only takes one tiny bit of $#it to gum up the works! I meant to say that the injector was fouled open, not completely open.

I had the valve covers off very recently(since the spray began), and checked things out, but didn't see or feel anything out of the ordinary there. The valve train sounds normal with a stethoscope as well. The injector in question pops as advertised, but it could very easily have a tiny piece of rubber stuck in it.

Thanks for the tip on Motec diagnostics! The Motec is something I'll be learning on a very slow curve! This would seem like the perfect time to begin playing with it. You guys are certainly the Motec experts, so I'll defer to your judgment.

I'll let you know what I find. Thanks again!


I did a compression test on all 8 today, and the cylinder in question had greater compression than the rest. A leaking intake gasket would appear to be the most likely cause. The condition of the spark plug points that way too. I had an oil leak from a manifold bolt directly in front of that throttle body. When that was going on, the gasket probably became soft, and blew out(was sucked into the cylinder). Sounds plausible to me anyway(I'm hoping it's that simple!)!
At least a stuck valve has been eliminated as a possible cause. Thank God!

I have been running with my air filter off (while playing with throttle linkage). I have noticed the window above the throttle bodies is occasionally dirty, usually over one or maybe two barrels, where reversion spits a little air/fuel mixture out the top of the velocity stack. Cam timing, engine speed, and throttle opening amoung other things will/may cause a "fog" of fuel above the butterflies.
Also, if the engine occasionally runs a little lean, it may pop back and blow the air/fuel mixture up onto the window.

Hi Dave,

I wish the fog were all that's involved. After the fog began to appear, the engine seemed to take a few more cranks to start. When I turned the engine over with no plugs, the compression tester (spark plug hole fitting) became wet with more than just gas(oil), when plugged into the offending cylinder(#8). The #8 plug showed signs of oiling as well. If not for the change in start up attitude, and the spray beginning at the same time as my manifold oil leak, I would have chalked it up to one of the things that you mentioned. The spray on the rear glass is also a bit thicker than raw gasoline.
It sounds like a bad or clogged injector. If it was a intake manifold leak it would run lean.

Swap the suspect injector with one from another cylinder and see if the problem goes to that cylinder.

Most injectors have a little screen in their intake. Debri rarely causes them to stick open, but will cause them to have low flow. If the injector is leaking it's probably bad.

Slim chance something on the control side is keeping the suspect injector open too long. You can listen to injectors with the stethoscope also.
Hi Kalun,

A stuck injector was my original thought, and is the next thing on my list to check. I'm going to do a leakdown test on the rails, to determine if the injector is staying open before replacing the manifold gaskets. Swapping injectors isn't a bad idea either! But, if it is an injector, what would account for the greater compression in the bad cylinder, the oiled plug, and the thicker spray residue? The injector sounds identical to the other 7. I agree with you on the lean condition with the manifold leak, but there are these other factors to consider. At this point, I'm just "happy as a pig in $#it", that it's not a stuck valve!
Thanks for your input!

Hi again -

If the compression is high, perhaps oil is getting into that cylinder? a failing valley to inlet gasket/seal? or perhaps head gasket??(any water in oil? or bubbles in header tank?) /ubbthreads/images/graemlins/confused.gif

How much higher was it? what were the figures?
Hi Paul,

The higher compression (15 lbs higher/the other 7 were within 2-3 lbs of each other) led me to believe that there was oil leaking into the cylinder. The intake(valley)gasket is what I suspect. The plug showed signs of oiling, and when turned over with no plugs, the "gas" that was expelled, seemed "thicker" than normal. Fortunately, there's no water or gas in the oil, or bubbles in the header tank. I did a fuel rail leakdown test, with the fuel line plugged after the rails. It held pressure just fine(not an injector). I have the new gaskets waiting, and hope to get them changed this weekend. If that doesn't cure the problem, it's back to the drawing board!


It sounds like you've got the demon chased into a corner. If you once again suspect an injector problem and your injectors are either 24 or 36 lb., let me know and I will send you a couple to try in place of the suspect injector(s).


I certainly appreciate your offer of an "injector loan", but

I am running 33lb injectors.

I'm not thinking beyond the intake gasket being at fault

for the moment. If that doesn't fix the problem, then I'll

look elsewhere. With no rail pressure leakdown, I doubt

that the injector is at fault. Time will tell........


Howard Jones

GT40s Supporter
Something to think about, A very high air fuel ratio(leaking intake) will not fire. This will apear to be a rich conditon on the plug. I had this exact problem once myself. It was a leaking intake gasket on the lifter valley side.

That's exactly what was wrong. The gasket had a break on the
lifter valley side. I glued everything back together this morning, test drove it about 30 miles, and never had any spit on the rear glass. It started easier and ran a lot better too!