Fuel Injector Flow Data: What 4 Years of Bench Testing Shows

May 8, 2026

By Ryan Bates, Marine Mechanic

Most of what's written online about fuel injector cleaning is opinion or marketing copy. We wanted to put numbers behind it. Since 2022, when we started keeping our flow bench data in a parsable format, we've tested 1,990 individual fuel injectors across 326 service jobs. This post is the data, with the rough edges left in.

A note on scope before the numbers: the 1,990 figure is what's in our digital records since 2022. We've been doing injector work for years before that, but those records weren't structured in a way we could analyze. So when we say "1,990 injectors," we mean specifically the bench data we can pull and verify, not lifetime work.

How We Test

Every injector that comes through goes on the same bench, a GB800 Multi-Port Fuel Injector Tester. We measure four things on every injector, before and after cleaning:

• Resistance (ohms) - tells us if the solenoid coil is healthy
• 10-second open flow (ml) - injector held open continuously, measures total flow capacity
• 60-second pulsed flow (ml) - injector pulsed at duty cycle, measures real-world fueling pattern
• Leak test (pass/fail) - injector pressurized closed, checks if seat seals properly

Test pressure varies by application. Standard MPFI runs at 40 psi. Some setups use 45 or 50. GDI direct injection runs at 100 psi or higher. TBI testing happens around 15 psi. The pressure tells us a lot about what kind of injector we're looking at, even when no part number is recorded.

About 1 in 3 Sets Arrive Out of Spec

The most useful number in our data is set-level imbalance - how far each injector deviates from its set's average flow. For an engine to run smoothly, all injectors in the set should flow within about 5% of each other. More than that and you start getting noticeable misfires, rough idle, and uneven combustion.

Of the 264 sets we have complete before-and-after data on:

• 35% arrived worse than 5% imbalance (out of spec by industry standards)
• 26% arrived worse than 10% imbalance (significantly out of spec)
• The average set arrived with 12% maximum deviation
• The median set arrived with 3% maximum deviation

What that means in plain terms: about 1 in 3 customers sending us injectors have a legitimate balance problem. Their engines aren't going to run right with what they have. The other 2 in 3 are sending in injectors as preventive maintenance, where the variance is smaller but the buildup is still affecting absolute flow.

Cleaning Brings Most Sets Back Into Spec

After cleaning, the imbalance numbers shift dramatically:

• 7% of sets remained worse than 5% imbalance (down from 35%)
• 4% remained worse than 10% imbalance (down from 26%)
• Average set imbalance dropped from 12% to 2.9%
• Median dropped from 3% to 1.5%

Translation: cleaning brings about 80% of out-of-spec sets back into spec. The 7% that don't recover are usually injectors with internal damage, worn pintles that won't seal evenly, or electrical issues that flow testing alone won't fix.

Per-Injector Flow Change (Why Balance Matters More Than Absolute Flow)

Before getting into the per-injector numbers, an important point: balance between injectors matters more than absolute flow. An injector that comes off the bench flowing slightly less than it did dirty, but is now matched to the rest of its set, is doing a better job for the engine than it was before. The engine doesn't care whether all injectors flow 75ml or 80ml - it cares that they all flow the same.

With that framing, here's the actual distribution across 1,720 injectors with matched before-and-after data:

• 33% had significant flow recovery (>5% increase) - heavily clogged injectors getting unblocked
• 43% had moderate flow recovery (1-5% increase) - typical buildup being removed
• 15% changed less than 1% - already clean, no buildup to remove
• 8% read slightly lower after cleaning - usually because they were the high-flow outliers, now brought into balance with the set

That last category (8%) sometimes confuses people, but it's not a problem. Some injectors flow high because of debris holding the pintle slightly open, or because of buildup that affected the calibration. Cleaning resets them to their true base flow rate. If they were the high-flow members of a set, they end up matching their lower-flow siblings instead of pulling ahead. That's exactly what flow matching is supposed to do.

The real metric to watch is the imbalance chart above, not absolute flow numbers. By that measure, cleaning brings 80% of out-of-spec sets back into spec.

Smaller Injectors Show Bigger Percentage Gains

One of the more surprising patterns in the data: smaller-flow injectors show much larger percentage improvement than bigger ones. This isn't because they're better candidates for cleaning - it's geometry.

Flow Class	Avg Improvement
Tiny (under 25ml/10s, small motorcycle)	+92%
Small (25-35ml/10s, economy 4-cyl)	+43%
Medium (45-55ml/10s, standard MPFI)	+3%
Large (65-75ml/10s, V8/marine)	+3%
Very large (75-90ml/10s, marine outboard)	+3%

The math: a partial blockage on a small injector with a tiny orifice represents a much larger percentage of total flow than the same physical blockage on a bigger injector. So when cleaning unblocks them, percentage improvement looks huge. Absolute improvement (in milliliters) is similar across sizes.

The practical takeaway: small motorcycle and economy 4-cyl injectors benefit dramatically from cleaning when they've been neglected. Bigger marine and V8 injectors still benefit, but in smaller percentage terms.

GDI vs MPFI: Different Cleaning Profiles

The data clearly separates GDI direct injection from standard MPFI in how they respond to cleaning:

Metric	MPFI (1,883 samples)	GDI (76 samples)
Average resistance	12.3 ohms	1.2 ohms
Avg flow improvement	+12%	+3.5%
Pre-clean leak fail rate	5.9%	9.2%
Post-clean leak fail rate	2.1%	1.3%

Why the difference? GDI injectors fire directly into the combustion chamber. The deposits they accumulate are different from what a port injector sees - mostly tip coking from combustion heat rather than internal varnish. Cleaning helps with the leak side dramatically (because GDI seat wear and varnish get cleaned up) but doesn't restore as much flow.

This matters if you're a GDI owner deciding whether cleaning is worth it. The honest answer: it helps with leak issues and partial restoration, but you should expect smaller flow gains than a port-injected friend would see.

About 10% of Injectors Are Beyond Cleaning

Not every injector can be saved. Out of all the resistance readings we've taken, about 9.9% are out of spec - either below 5 ohms (often shorted coils) or above 25 ohms (often open coils or damaged windings). Those are electrical failures and no amount of ultrasonic cleaning will fix them.

We've measured resistance values from 1.0 ohm (likely a shorted high-impedance coil) up to 77 ohms (likely an open coil or severely damaged winding) when we should have seen 12-14 ohms. Those injectors get flagged on the report and we let the customer know before shipping back.

On the leak side, about 2% of injectors still fail leak testing after cleaning. Those usually have internal seal degradation, worn pintle seats, or cracked bodies that no chemical or ultrasonic process can address.

Combined, roughly 10-12% of injectors that come through our shop are honestly beyond cleaning and need replacement. The flow bench is what tells us which ones.

What We Mostly Service

About 95% of our work is marine outboards. Most of that lands in a small handful of injector families we know cold:

• CDH210 / INP-771 — the shared OEM injector across Yamaha F75-F150 and Suzuki DF90-140. Our most common job by a wide margin.
• CDH275 / EAT152 — Yamaha F150 4-stroke, 2004 onwards.
• Keihin OX66 — Yamaha 250HP V6 2-stroke EFI.
• Mercury Optimax — V6 with paired fuel and air injectors. We service the full set.
• Yamaha HPDI — 2-stroke direct injection, yellow and orange flow ratings.

The non-marine work is mostly performance: Harley-Davidson (IWP069 Magneti Marelli), Mercruiser inboards, and the occasional powersport. We do basically zero general automotive cleaning - if you have a regular passenger car needing injectors cleaned, you're better off with a shop that does that as their main work.

What This Data Means If You're Considering Cleaning

A few honest takeaways for somebody on the fence about whether to send injectors in:

If your engine is misfiring or running rough - you're probably in the 1-in-3 group with real flow imbalance. Cleaning has an 80% chance of bringing your set back into spec.

If you're doing preventive maintenance with no symptoms - cleaning will probably show modest improvement (a few percent on bigger injectors). Worth it if you're rebuilding or repowering, less essential if everything is running fine.

If you have a GDI engine - cleaning helps but expect smaller flow gains than the marine outboard guys see. The bigger benefit is the leak test results.

If your injectors might have electrical issues - bench testing is the only way to find out. Resistance readings tell us within seconds whether an injector is electrically healthy. About 10% of what comes through can't be saved, and the bench data tells us which one.

The flow report we ship back with every order shows you all of this for your specific set. Here's what we look for when telling a healthy injector from a bad one, and why bottle additives can't replicate what a flow bench shows you.