# Diesel Fuel Injector Repair in Upland > Diesel fuel injector repair in Upland for rough idle, knocks, smoke and fuel in the oil. We test balance and return flow before replacing injectors. Source: https://route66autorepairshop.com/diesel-engine-services/diesel-fuel-injector-repair Updated: 2026-09-17 Business: Route 66 Auto Repair, 1775 W Foothill Blvd, Upland, CA 91786, (909) 608-7699 Hours: Monday to Friday 8 AM–5 PM, Saturday 8 AM–2 PM, Sunday closed **TLDR:** We confirm the failed injector with **balance and return-flow tests**, replace or repair only what the data supports, and check for **fuel in the oil**. Diesel fuel injector repair at Route 66 Auto Repair in Upland starts by finding which injector is actually at fault. A worn or sticking injector can cause a rough idle, a rattle, smoke, hard starts or fuel diluting the engine oil. Our ASE-certified technicians check correction values and return flow, then quote only the injectors the tests point to, on a written estimate. ## What are the signs of a failing diesel injector? - A rough or lopey idle that smooths out at higher engine speed - A sharp rattle or knock from one side of the engine - Black smoke under load or white smoke at start-up - Hard starting after the truck sits, with fuel pressure slow to build - Rising oil level on the dipstick or a fuel smell in the oil ## How do we find the bad injector? The engine computer trims fuel to each cylinder to keep the engine smooth. When one injector wears, its trim value drifts away from the others. We read those values at idle and under load, then run a return flow test on engines that allow it, which shows an injector leaking back more fuel than it should. A cylinder contribution test tells us which cylinder is not pulling its share. ## Our diesel injector repair process 1. **Test and isolate:** Balance values, return flow and contribution tests identify the injector or injectors at fault. 2. **Estimate the right scope:** You see whether one injector, a bank or a full set makes sense, with reasons for each option. 3. **Replace with new seals:** Injectors go in with new sealing washers and hold-down hardware torqued to spec. 4. **Code and prime:** Where the engine requires injector codes, we enter them. The fuel system is primed and checked for leaks. 5. **Verify:** We recheck balance values and road test before the truck leaves. ## What's included in diesel injector repair - Injector balance and return flow testing - Oil condition check for fuel dilution - Injector replacement with new seals and washers - Injector coding where the engine requires it - Fuel system priming and leak check - Post-repair data review and road test ## Is a bad injector need-now or need-later? **Need now:** - Fuel diluting the engine oil - A loud knock or heavy smoke under load - An injector leaking externally near the head **Need later:** - One correction value drifting but inside the maker's limit - A slight idle roughness with no smoke or dilution ## How pricing and timing work Price depends on the engine, how many injectors need replacement, and how much has to come off to reach them. Whether you choose new or remanufactured injectors also changes the number, and your written estimate shows every step. Many single-injector jobs finish within a day when the board allows. AAA members save 10% on parts and labor, up to $75. ## Aftercare Expect a smoother idle once the new injector learns its trim over the first few drives. Check for fuel smell around the engine for a week. Changing fuel filters on time protects the new parts; see [diesel fuel filter replacement](https://route66autorepairshop.com/diesel-engine-services/diesel-fuel-filter-replacement). If deposits rather than wear were the issue, [diesel injector cleaning](https://route66autorepairshop.com/diesel-engine-services/diesel-injector-cleaning) may be the lighter option next time, and a failed copper washer is covered under [diesel fuel injector seal replacement](https://route66autorepairshop.com/diesel-engine-services/diesel-fuel-injector-seal-replacement). ## Frequently asked questions ### Should I replace all diesel injectors at once? Not automatically. If one injector failed early and the rest test well, replacing one is reasonable. On high-mileage engines where several values are drifting, a full set can cost less than repeat visits. We show you the test numbers so you can choose. ### Can a bad injector damage the engine? Yes. A stuck-open injector can wash oil off a cylinder wall, dilute the oil and, in the worst case, damage a piston. That is why fuel in the oil moves an injector job to need-now. ### Do new injectors need programming? Many modern diesels store a calibration code for each injector. When the engine requires it, we enter the code so the computer fuels the new injector correctly. We tell you up front if your engine has any coding steps. --- Call (909) 608-7699 or visit https://route66autorepairshop.com/contact for a written estimate.