Cracked Cylinder Head Symptoms in Diesel Engines
The most common cracked cylinder head symptoms on a heavy-duty diesel are coolant loss with no outside leak, white smoke, bubbles or pressure in the surge tank, overheating, and a misfire or low compression on one or two cylinders. A blown head gasket, EGR cooler, injector cups or O-rings, oil cooler and air compressor fake the same signs, so test before you pull the head.
A cracked head on a Class 8 truck rarely announces itself. There's no bang and no fault code that says cracked head. What you get is a coolant level that keeps dropping, a driver who says it runs fine, and an argument on the shop floor about whether it's the head, the gasket or the EGR cooler. We run a cylinder head shop in Houston, and pulling a head is a big labor job, so don't do it on a guess. Here's what the symptoms look like, what else causes them, and the order to test in.
What are cracked cylinder head symptoms on a heavy-duty diesel?
Seven signs show up again and again. None of them proves a crack on its own. Together they tell you where to test.
1. Coolant loss with no puddle under the truck
This is the most common complaint and the one that gets ignored longest. Coolant disappears with nothing on the ground and nothing on the frame rail. A level that keeps dropping with no visible leak means an internal leak, either a cracked head or a gasket (guide to diagnosing cylinder head failures). If a driver is topping off the surge tank on a schedule, that's a diagnosis in progress, not a maintenance item.
2. White smoke and a sweet coolant smell
Coolant burning in a cylinder makes white smoke and leaves a sweet coolant smell in the exhaust or the engine bay. Write down when it smokes and for how long. One Series 60 owner reported white smoke for about a minute after a cold start that then cleared, with bubbles and pressure in the surge tank at idle and no coolant in the oil (Series 60 cold-start report). That pattern matters when you separate head, gasket and cooler.
3. Bubbles, pressure or coolant pushed out the overflow
A crack into a coolant passage lets combustion gas into the cooling system. You see bubbles in the surge tank, hoses that go hard, or coolant pushed out the overflow, and it can happen with a normal temperature reading. One ISX owner reported coolant pushed from the reservoir into the overflow, worst pulling hills, with no overheating. The cause was a cracked head (ISX coolant loss thread, TheTruckersReport).
4. Overheating that keeps coming back
Overheating is both the main cause of cracked heads and a symptom of one. Gas pushing coolant out of a cracked passage creates hot spots, and hot spots make the crack grow (signs a diesel cylinder head needs replacing). If the gauge keeps climbing after parts swaps, check for combustion gas in the coolant before you buy another thermostat.
5. Coolant in the oil
Milky oil, or glycol on an oil report, means coolant and oil are meeting somewhere. The head is only one suspect: oil coolers, liners and gaskets do it too. Don't keep running it. Glycol reacts with the oil's detergent and forms gels and sludge that plug filters (Machinery Lubrication), and the advice from Series 60 mechanics is to sample the oil and park the truck before the main bearings spin.
6. Fuel in the coolant, or coolant in the fuel
A film of diesel on the surge tank, or bubbles in the fuel tank, means the fuel and cooling systems are connected somewhere. A crack can do it, and a crack that reaches a fuel passage can also put fuel in the oil. On the ISX, ISX15, 3406E and C15, though, injector cups, seats and O-rings are the first suspects, covered below.
7. Misfire, rough idle or low compression on one or two cylinders
Gas escaping through a crack costs compression, so you get poor performance, a rough idle or a weak cylinder. A cutout or compression test that shows one or two low cylinders tells you where to look.
What else causes the same symptoms?
Seven other failures fake a cracked head, and most are cheaper and quicker to fix. The table pulls together what mechanics report on these engines. These are field reports, not OEM bulletins, so use it as a checklist, not a verdict. Check the cheaper causes first.
| Look-alike | Where it's reported | What it fakes | How to rule it out |
|---|---|---|---|
| Head gasket | All families | Gas in coolant, coolant loss, white smoke | Watch when pressure builds (next section). With the head off, inspect the old gasket for tracking. On a Series 60, check the counterbore. |
| EGR cooler | ISX, EGR-era Series 60 | Coolant loss with no leak, pressure in the cooling system | Test the cooler on its own before the head comes off, following the OEM procedure. |
| Single-cylinder air compressor | Series 60 | Pressurizes the cooling system like a failed gasket | Bypass the compressor and retest. |
| Injector cups, retaining hardware, O-rings | ISX, ISX15 | Coolant in fuel, bubbles in the fuel tank | Replace the O-rings first. If the bubbles come back, pull and test the head. |
| Injector seat leak | 3406E, C15, C15 ACERT | Fuel in coolant plus a miss | Inspect the injector and cup O-rings for burn damage and check the seat. |
| Oil cooler | 3406E, C15 | Oil in coolant, coolant in oil | Test the oil cooler before condemning the head. |
| Cylinder liners | C15, ISX | Coolant in oil | Look for pitting, and for coolant running between piston and liner during a cooling system pressure test. |
The Cat injector seat deserves a closer look. The 3406E, C15 and C15 ACERT share the same head, injectors and injector cups, and the injector seats directly against the head casting, not the cup. If that seat leaks, combustion pressure burns the injector and cup O-rings, and you get fuel in the coolant and a miss that looks just like a crack (3406E and C15 fuel-in-coolant explainer). On a 3406E with oil in the coolant, one technician called a cracked head quite rare and pointed to the oil cooler first (3406E oil-in-coolant thread).
One caution: you'll read on pickup forums that injectors can't leak coolant, so coolant at the injector means a cracked head. That was said about the 12-valve Cummins. It doesn't carry over to the ISX or C15, where cups, seats and O-rings really do leak.
Is it a cracked head or a blown head gasket?
Often you can't be certain until the head is off, but when the pressure builds is a useful clue. Mechanics on Series 60 forums report that these heads tend to crack in the intake port just above the valve seat, and a crack on the intake side only builds cooling system pressure under boost. A failed head gasket builds pressure any time the engine runs (Series 60 techs thread). Their check: pressure-test the cooling system, pull the intake manifold, run the engine and look for coolant in the intake ports.
Two more points from the field. On the Series 60, head gasket failures are often traced back to a failed counterbore, so a new gasket on a bad counterbore buys you a comeback (Series 60 head-or-EGR thread). And in the ISX case above, the same symptom came back after a new head went on. Replacing the head doesn't end the diagnosis; confirm the cooler, compressor and liners are clean too.
How do you confirm a cracked head before you pull it?
Run these in order. Each step rules something out, and the cheap ones come first.
- Pressure-test the cooling system following the OEM procedure, and watch the surge tank for bubbles or pressure rise. Pressure that drops with nothing on the ground points internal.
- Rule out the look-alikes. Bypass the air compressor, test the EGR cooler and oil cooler, and check the injector cups and O-rings, using the table above.
- Run a combustion gas test at the surge tank. A combustion leak tester, often called a block tester, checks the air above the coolant for combustion gas (combustion gas test overview). A positive result proves gas is in the coolant. It doesn't tell you whether it came from a crack or a gasket.
- Pull an oil sample. What to look for is below.
- Run a cutout or compression test. Low compression on one or two cylinders narrows the location to head, gasket or liner on those holes.
- On a Series 60, pull the intake manifold and run the engine to look for coolant in the intake ports, as described above.
- Only then pull the head and test the casting properly.
What an oil sample tells you
An oil sample is the cheapest early warning you have. Glycol, along with sodium, potassium and boron from the coolant additives, points to a coolant leak. The lab can't tell you which part is leaking: a blown head gasket, a cracked head or block, a cooler core and liners all show up the same way (Machinery Lubrication; Chevron oil analysis guide).
Read the report as a whole. High potassium by itself doesn't always mean glycol (Machinery Lubrication on oil analysis tests), so look for the elements and glycol together and watch the trend across samples. Coolant contamination isn't rare: one heavy-equipment lab cited by Noria found glycol in 8.6% of motor oil samples, about 1 in 12 (Machinery Lubrication on glycol in oil). If glycol shows up, stop running the engine until you find the source. A spun bearing turns a head job into an overhaul.
Where do heavy-duty diesel heads crack?
Cracks start where heat and stress concentrate: around the valve seats, the injector bores and the coolant passages. Overheating is the main cause. Uneven expansion stresses those areas, and even a single overheating event can weaken the head. Casting stress points and installation problems are the other common causes (common causes of diesel head cracking). Repeated heating and cooling can also open up flaws already in the casting (cylinder head pressure testing overview).
Cracks often run between the valves, but not always. By family, here's what the field reports say:
- Detroit Series 60 DDEC V: forum mechanics report cracks in the intake port just above the valve seat, which is why the boost-only pressure pattern matters.
- Cat 3406E, C15 and C15 ACERT: same head, injectors and cups across all three, so the injector seat leak has to be ruled out before you call a crack.
- Cummins ISX and ISX15: for coolant loss, mechanics list the head gasket, EGR cooler, liners and DEF injector alongside the head. For coolant in fuel, it's usually injector cups or retaining hardware, or a cracked head.
How should the head be tested once it's off?
Clean it, pressure-test it with the water jackets blanked off, and test it hot. Shops that do this work heat the head or test it in warm water, because a cold pass isn't conclusive (Magnaflux vs. pressure testing discussion; guide to cylinder head pressure testing). Shops quote different test pressures and none of them are OEM figures, so take the test pressure and procedure from the OEM service manual.
A pressure test tells you whether it leaks. Magnetic particle inspection (Magnaflux) or dye penetrant tells you exactly where (testing of cylinder heads). Pressure testing is recommended after any coolant leak, overheating event or mixing of fluids (cylinder head pressure testing service page), so test the head even if the gasket turns out to be the failure. If someone tells you a used head is good and can't say how it was tested, assume it was looked at, not tested.
Should you repair or replace a cracked head?
Replace a cracked head. A head with a warped deck and no cracks may be a candidate for resurfacing, but the flatness limit and minimum deck thickness come from the OEM service manual for your engine, so have a machine shop measure it before you decide. For a casting that has cracked at the fire deck, between the valves, in the ports or at the injector bores, the dependable fix on a working truck is a replacement head.
Here's how the options compare under TTM's warranty for engine components (full terms on our warranty page):
| Option | TTM warranty term |
|---|---|
| New loaded head (valves, springs, seals and guides installed, ready to bolt on) | 12 months against defects in material and workmanship unless the product page says otherwise; no core charge |
| Remanufactured | 12 months |
| Reconditioned | 3 months |
| Good used, tested and inspected | 30 days |
A used or remanufactured casting has already been through years of heat cycles. A new casting starts with none. TTM's new heads come in stages: Stage 1 has standard OEM-style valves, Stage 2 adds upgraded Inconel exhaust valves, and Stage 3 uses Inconel intake and exhaust valves. Examples: the new Cat C15 cylinder head, 2239250 (3406E and C15 ACERT universal), the new fully loaded Detroit Series 60 DDEC V 14L cylinder head, 23538858, and the new loaded Cummins ISX15 cylinder head, 4374091.
What should you replace while the head is off?
Pulling the head is the expensive part. Skimping on the hardware that goes back with it is how you do the job twice.
- Head bolts. On torque-to-yield designs, head bolts are single use. Fit a new kit, such as the Cat C15 head bolt kit, 1241854, and take the torque sequence and stage values from the OEM service manual. Scrap the old bolts.
- Gasket set. Use a complete set for the engine, for example the Detroit Series 60 head gasket set, 23532333 or the Caterpillar 3406E head gasket set, 2161252.
- Spacer plate on the Cat. Check the plate before it goes back on. The Cat C15 spacer plate, 6I4421 is offered in standard and undersize thicknesses; which one you need comes from the measurements in the OEM procedure.
- Coolant and oil. Flush the cooling system, because whatever leaked into it is still there. If coolant reached the oil, change the oil and filters and resample after the repair.
How do you keep the new head from cracking?
Fix the cause before the new head goes on. Overheating is the main reason heads crack, so a new head on a cooling system with a weak fan, a plugged radiator or a low coolant habit will crack too. Find out why the old one overheated, and run the cutout, oil sample and cooling system pressure test again after the repair to confirm it's fixed.
It also protects your warranty. TTM's warranty doesn't cover damage from improper installation, inadequate cooling, fuel contamination, over-fueling, overheating, low oil or coolant, skipped break-in or non-OEM fluids. Labor, diagnostics, removal and installation, towing, freight and downtime aren't covered either. If something goes wrong, notify TTM within 72 hours of detecting the issue, and don't tear the engine down until TTM authorizes it in writing.
Frequently asked questions
Can you drive a truck with a cracked cylinder head?
Not for long, and not loaded. A crack lets combustion gas push coolant out of the passages, which creates hot spots, and hot spots make the crack grow. If coolant reaches the oil, glycol forms gels and sludge that plug filters, and the next risk is spun bearings. Pull an oil sample, stop running it and diagnose it before a head job turns into a bottom-end job.
How do I know if it's the head or the head gasket?
The symptoms overlap almost completely, and often you can't be certain until the head is off. Mechanics on Series 60 forums report that a crack in the intake port builds cooling system pressure only under boost, while a failed gasket builds it any time the engine runs. Once the head is off, inspect the old gasket for tracking and pressure-test the head anyway, because a failed gasket doesn't clear the casting.
Will a block tester find a cracked head every time?
No. A combustion leak tester detects combustion gas in the coolant, so the crack has to be open between a combustion chamber and a coolant passage while you test. A crack into an oil or fuel passage won't show, and Series 60 mechanics report intake-port cracks that only push gas into the coolant under boost, so a test at idle may read clean. A negative result doesn't clear the head; move on to the cutout test and an oil sample.
Can a cylinder head pass a pressure test and still be cracked?
Yes. A head tested cold can pass and still leak in service, which is why shops that do this work test with the head heated or in warm water, and use magnetic particle or dye penetrant inspection to find exactly where a crack is. Ask how a used or rebuilt head was tested before you trust a pass, and take test pressures and procedure from the OEM service manual, since shops quote different figures.
Is coolant in the fuel on a Cummins ISX a cracked head or injector cups?
Usually the cups, retaining hardware or injector O-rings, not the head. One ISX mechanic put it at about 90 percent O-rings and 10 percent cracked head, so replace the O-rings first; if the bubbles in the fuel tank come back, pull the head and test it. On a Cat 3406E or C15, fuel in the coolant with a miss usually means the injector seat against the head casting is leaking and has burned the O-rings, not a crack.
Why is my diesel pushing coolant out the overflow with no overheating?
Combustion gas or compressed air is getting into the cooling system and pushing coolant out, and that can happen with a normal temperature reading. One ISX owner saw it worst pulling hills and the cause was a cracked head, but on a Series 60 a single-cylinder air compressor or an EGR cooler does the same thing. Pressure-test the cooling system, bypass the compressor, test the cooler and run a combustion gas test before you blame the head.
How much does it cost to replace a diesel cylinder head?
It depends on the engine family, whether you buy new, remanufactured, reconditioned or used, and your shop's labor rate and book time. Budget for the head, a complete gasket set, a new head bolt kit on torque-to-yield designs, a coolant flush, an oil change if coolant reached the oil, and the downtime. Get book time from your shop and check current part pricing on the product page, for example our new Cat C15 cylinder head.
Is a new cylinder head worth it over a remanufactured one?
It comes down to the truck's remaining life and the warranty you want behind the part. A remanufactured head is a used casting that has been through years of heat cycles before it was inspected and machined; a new casting starts with none. TTM's new loaded heads carry a 12-month warranty against defects in material and workmanship unless the product page says otherwise, with no core charge; remanufactured heads also carry 12 months, reconditioned 3 months, and good used 30 days.
Sources
- 4 Common Causes of Diesel Cylinder Head Cracking (Highway and Heavy Parts)
- 4 Signs You Need to Replace Your Diesel Engine Cylinder Head (Highway and Heavy Parts)
- 3406E / C15 Fuel in Coolant and Engine Miss Explained (Highway and Heavy Parts)
- Signs You Need to Replace Your Cylinder Head (DieselPro)
- In-Depth Guide to Diagnosing Cylinder Head Failures (Choate Performance)
- Cummins ISX puking coolant (TheTruckersReport forum)
- Cummins ISX coolant loss (TheTruckersReport forum)
- 07 International Cummins ISX (TheTruckersReport forum)
- ISX15 coolant in fuel (JustAnswer)
- Detroit Diesel Series 60 techs question (Yellow Bullet forum)
- Detroit 60 bubbles and pressure in coolant: head or EGR (TheTruckersReport forum)
- 2006 Freightliner Detroit Series 60 cooling system pressure (JustAnswer)
- Detroit 60 bubbles and pressure in overflow coolant reservoir (Diesel Truck Resource forum)
- 3406E oil in coolant, oil cooler tested (JustAnswer)
- C15 head gasket and oil cooler (JustAnswer)
- 3406 C15 coolant in oil (Diesel Engine Trader)
- Sodium and potassium in oil (Machinery Lubrication)
- Glycol in oil (Machinery Lubrication)
- Oil analysis tests (Machinery Lubrication)
- Interpreting oil analysis (Chevron Lubricants)
- Testing of cylinder heads (Centar Glava Motora)
- Magnafluxing vs pressure testing (Camaros.net forum)
- A guide to cylinder head pressure testing (Merdeka Precision)
- Cylinder head pressure testing (Powermax Engineering)
- Cylinder head pressure testing (Conso)