I have a 1994 Ford F-150 4.9 L6 that I have fixed up and would like to DD. When I drive it long enough to get to operating temperature, I will eventually get a headache that is similar to the headache I have gotten in the past when welding in a poorly ventilated area.

The exhaust system has been checked several times via bubble test. There are NO leaks. The shop vac can barely run because of the back pressure when hooked up the the tailpipe.

There is a very small oil leak on one corner of the pushrod cover, which I will be fixing. Other than that, there is no reason to believe that this truck has any source of fumes.

The clutch was replaced recently. I used to be able to smell it burning at highway speeds. That is gone.

If it's not the pushrod cover leak, what else could it be? I've heard of interior plastics and glues outgassing, is that possible on this truck? It has a rubber floor, from the factory.

Any other ideas appreciated.

  • 12
    Put a carbon monoxide sensor in the cabin when you drive, see if it alarms.
    – Moab
    Aug 7, 2016 at 15:18
  • Do you have any power loss or have you noticed lower fuel efficiency?
    – Jason C
    Aug 7, 2016 at 18:16
  • Btw here's another interesting technique to find exhaust leaks that may be easier than a bubble test: youtube.com/watch?v=Fb90LPj41cw
    – Jason C
    Aug 7, 2016 at 18:27
  • 1
    If you haven't checked this, you might have a crack that only opens with the engine hot.
    – George
    Aug 8, 2016 at 10:48

7 Answers 7


try checking the exhaustion, 99.99% of nausea related-car come from bad exhaustion especially with Ford manufacturer.

  • This is in fact the correct answer. The exhaust system sprung new leaks, in the same places.
    – Craig R
    Nov 28, 2016 at 0:24

The shop vac can barely run because of the back pressure when hooked up the the tailpipe.

If there is a severe exhaust restriction it could explain the presence of fumes in the cabin:

  • if the engine is running the exhaust gases have to escape from somewhere, regardless of whether the exhaust is free-flowing or not. If your vehicle employs an EGR setup that would be one way for the engine to continue running despite having a clogged exhaust.
  • EGR plumbing isn't designed for very high pressures; it is reasonable to assume that some exhaust gas will vent out to the engine bay from hose connections, cracks, etc.
  • once the exhaust gases are in the engine bay, the cabin air intake will have no problem sucking it into the cabin
  • Sounds reasonable, at the very least. Aug 7, 2016 at 17:58
  • I doubt there is any kind of exhaust restriction; the engine has good power (so to speak) and returns expected fuel economy.
    – Craig R
    Aug 8, 2016 at 1:38

Seeing as how the vehicle is over 20 years old, I'm doubting you are getting outgassing. That should have occurred a long time ago. If you drive with the window open to ensure fresh air flow, does the headache still occur? Maybe you just have a psychosomatic reason and your head just starts hurting because deep down you hate the truck (really just kidding here). Exhaust would be the only factor here which I can think of.

Leaking anything, unless it's enough to burn and cause fumes, would not be causing this issue. You may try power washing all of the engine bay space to remove any oil which may be present. If you could then drive the vehicle for an appreciable amount of time before the headache returns, it would tell you the leaks could be causing your issue.

Beyond this, really there'd be no way to answer your question completely as we don't know your medical history (and we don't need to know it), nor are we physicians who can give you brain diagnostics for your issue (that is unless you have an OBDII port on the back of your head? J/K). I hope you can get it figured out.


Since you qualified the exhaust system has no leaks, check the erg tube for leaks, also check the exhaust manifold gaskets if it has them, between manifold and cylinder head.

Also check for oil leaks that drip onto exhaust manifolds, this will create CO and will get sucked into the passenger cabin.

Best way to check for exhaust system leaks (exhaust manifolds back to tail pipe) is to use a smoke machine connected to the tail pipe(s).

To check for hydrocarbons (Carbon Monoxide=CO) in the passenger cabin Put a carbon monoxide sensor (similar to what is used in houses and motor homes) in the cabin when you drive, see if it alarms.

EGR Tube image below.


EGR Tube

  • I checked the head to manifold mating, along with EGR and smog pump ferrules.
    – Craig R
    Aug 8, 2016 at 1:36

Check for rust holes in your cab. Even pinhole leaks can admit exhaust gases. Look for small holes in your firewall, or degraded rubber gaskets for cables, wires, etc., as they pass through the firewall. Check for rust holes in the floor, under the rubber matting. As a vehicle moves down the road, it creates a vacuum, and exhaust gases get sucked forwards, back under the truck. Any leaks in the exhaust system, no matter how small, can get sucked up into the cab. Check your exhaust manifold for a bad gasket, or hairline cracks. Listen closely for leak sounds at the manifold. Were the components of your exhaust system assembled using hi-temp exhaust sealant paste? Often, a seemingly airtight joint, will have small leaks. Both slip joints, and flanged joints, are prone to this problem. Often exhaust gases will slip through an open window, too. Drive with windows closed, and with windows open. Any difference in your headache? Is your exhaust system stock? Does the system end under the bed, or does it extend all the way to the back of the truck, as it should? Are the rolled assembly joints on the muffler itself, tight and leak-free? Outgassing of plastics is probably not an issue. The truck is too old. Listen for exhaust leaks with both a cold engine, and a hot engine. Expansion and contraction of metals, joints, etc., can open up or close down system leaks.


I had similar in my 1977 Volkswagen Polo and traced the problem to the combination of an overcharging alternator and a hole in the battery compartment meaning gasses from the battery were coming into the cabin.

There are a number of things which could be causing this including a leaking fuel tank and a leaking exhaust system. Please do treat this problem as urgent as becoming unconscious at the wheel whilst driving could have catastrophic consequences.


I noticed this in my truck, the flat bed causes a vortex that sucks exhaust fumes in through the windows. try covering the back with a tarp, it might change the dynamics. alternatively, you may have a really old bluetooth interface to the electronics or sound system , bluetooth causes a headache for me 100% of the time its turned on.

  • 2
    I can guarantee you it is not the Bluetooth. If it was you would be having a constant headache from any and all radio transmissions. The only connection that can explain this correlation is a nocebo effect (the reverse of a placebo effect). And sure, turning off the Bluetooth will help you there, but not because there is any physical effect of it.
    – MichaelK
    Aug 8, 2016 at 6:10

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