Whenever I’m pondering the possible cause of something like a hesitation/stumble concern, I find it particularly useful to draw on the experiences I received tackling drivability issues before fuel injection was the order of things. This especially is true because many, if not most, hesitation problems are fuel-delivery related, and for the most part, an engine is still an engine.
In the simple, yet complex world of carburetors, atmosphere is the primary driving force behind fuel delivery. There is a built-in fuel reservoir that has a needle-and-seat at the place where fuel comes from the pump, with that needle sitting on a float arm and its tiny cone-shaped stopper poised to staunch the flow of fuel as the float rises in response to the filling of that reservoir. Furthermore, that float has to be adjusted to stop the flow of fuel into the reservoir whenever the fuel level is at the right depth. If the reservoir level is too low, the demand might not be met; if the level is too high, unwanted fuel might begin to siphon and dribble into the intake through the venturi.
Creating and Breathing Fire
As the engine’s pistons and valves do their work, extreme low pressure is created below closed throttle plates (we call this intake vacuum). The venturi don’t get enough flow to draw fuel, so the carbureted engine is forced to get its idle mixture from the tiny amount of fuel that mists out of the little hole(s) under those throttle plate(s) through which the idle mixture screw’s sharp tip is protruding, and turning this screw one way or another changes idle fuel delivery and greatly impacts the quality of the idle. Fuel injected engines are the same, except that the fuel is atomized at the tip of the injectors, which point at the back of the intake valve. They don’t need adjusting, but they sometimes need cleaning.
When I’m troubleshooting a no-start on a carbureted vehicle, the first thing I do is hold the choke open for a visual while moving the throttle to look for those squirts. If I don’t see the squirts and the engine doesn’t start, I know there is no fuel in the carburetor. If, however, the engine runs but stumbles on acceleration, a weak or nonexistent squirt points to accelerator pump issues. When I’m looking at throttle body injectors, fuel delivery is visible too, and that’s a good thing.
On older vehicles, stumble/hesitation generally is related to the accelerator pump, but it also can be related to the thermostatic air cleaner system, which provides hot air through that funky foil hose from the exhaust manifold stove during cold operation to help with fuel atomization. Some modern fuel injected engines still have a thermostatic air cleaner for the same reason (fuel likes to condense rather than atomize in cold air and efficiency is directly tied to atomization of the fuel). That’s one reason Gasoline Direct Injection (GDI) is so efficient — it atomizes fuel to the Nth degree. Smaller droplets mean a better burn. Another reason GDI is better is the fact that the fuel charge can be modified on the fly in the way injection is timed during the stroke and even in the way it is delivered.
Weak spark and/or worn out spark plugs can cause stumbles too, and that’s a significant issue, particularly on so many distributor-equipped vehicles that sport a single coil. Check your scope trace for the absence of those reserve energy squiggles. That kind of ignition trace means the coil is weak.
Some engines with crank sensor trigger wheels on one of those looser-fitting balancers that don’t press on and off the crankshaft nose can stumble if the balancer retaining bolt isn’t tight enough, because the balancer eventually will wallow out the keyway, allowing the balancer and the trigger wheel to rock back and forth, changing position in relation to the crankshaft. This condition affects ignition timing in a big way on crank sensor equipped vehicles. Similarly, if a press-fit balancer isn’t torqued properly so as to pinch the crankshaft gear, the keyway can shear and cause the cam timing to change. That usually lowers engine vacuum and reduces power all the way around.
To start an engine, the accelerator pump’s job was, for a few years, done by a cold start injector mounted in the manifold on some platforms, but then those smart guys sitting at the laboratory keyboards developed the notion that the regular injectors could be manipulated to double pulse all at once during engine start so as to provide the necessary wetting of the intake manifold. The ignition timing is also computer-retarded to its base setting during starting so the engine won’t kick back during the start cycle (this timing retard was done manually with a steering wheel lever on 1920s vintage Fords) and then as the engine fires up, the timing is advanced for power and the injectors begin to fire sequentially.
Airflow and/or manifold pressure provide additional inputs as the driver demands it, but because of volumetric issues, those inputs alone aren’t enough to prevent a stumble when the go pedal is applied. As the throttle is opened, the PCM notes the speed and extent of throttle action and instantly responds on the systems you see with an O-scope by adding a few extra injector clicks in between the otherwise crank speed-cadenced pulses. This provides a very efficient “accelerator pump” increase in fuel delivery to help overcome the inertia, thereby preventing a stumble. If a throttle position sensor’s internal strata becomes worn to the point that signal dropout occurs during acceleration, a stumble will be the natural result. At best, a stumble of this nature is annoying, and at worst, the engine can stall. But that doesn’t usually happen on fuel injected engines. Chafing TP sensor wires obviously can trigger this kind of thing, too.
If the stumble in drive isn’t there in reverse, check the air inlet tube. I once encountered a Ford Probe that stumbled only in drive, like an air inlet tube crack because the harness was trying to yank the MAF connector away from the sensor on forward acceleration. Pay attention to wire harness chafing issues for the same reason. Wire harnesses are thick, and they stiffen with age, so they like to rub or burn holes in themselves as they nuzzle up to an exhaust manifold, EGR pipe or sharp bracket. I’ve seen wire harnesses on turn of the century F-series pickups dreadfully damaged because they were constantly rubbing on that smooth aluminum A/C suction line near the accumulator.
It turned out that someone had put diesel fuel in the tank before taking it back to the rental place, and I found it by smelling the fuel. I got some on my hand, waved it around until it should have been dry, noticed it was greasy and smelled diesel residue. That engine was, in addition to its struggle with extremely slow-burning fuel, experiencing a compression increase because of the perpetually greasy residue on the cylinder walls. Replacing the fuel took care of that one. Likewise, an engine platform that is tuned mechanically and electronically to run on 87 octane can snort and pop when it’s cold if somebody is using 93 octane fuel, because higher octane fuels burn slower. In addition to the cold stumbles high octane causes on platforms tuned for low octane gas, that higher octane fuel tends to leave some residue on the head and piston that eventually becomes carbon, effectively raising compression over time, and that raise compression eventually causes detonation/ping concerns.
Case in point, one of my fellow techs at the Ford dealer worked for about two hours on a new Ford Bronco back in the early 1990s that had an awful cold stumble until I told him to ask the customer what kind of fuel they were using. They told him they were using the most expensive 93 octane they could find, and when he heard that answer, his job was done. He told the very surprised customer to get a tank of 87 the next time they filled up (and forever after that) and save themselves some money.
In a nutshell, a successful stumble-finder will understand the basics of initial acceleration, thus recognizing the importance of consistently strong spark delivered at the right time, coupled with smooth transitory fuel delivery during that critical moment. Gathering and interpreting data is everything, and any anomalous sensor or airflow condition that coincides with these events is almost certain to cause hesitation on acceleration.
