Coolant exhaust gas test false positive

[tigaress]

I have this Defender in the garage right now. I am a little confused since the mechanic told me the head gasket was shot. The block test came up positive for exhaust gas in the coolant, so I kinda panicked. The thing is, before the test I ran a full flush using citric acid to clear some gunk. To double check, I pulled the heads and don't see any signs of failure.

Now I'm wondering... could residual acid from that flush skew the test? Are these block tester fluids sensitive to stuff like that?

 

[Patataaa_]

Yeah, a block test can definitely be sensitive to residual chemicals.
I wouldn't be surprised that citric acid, being an acid, could potentially react with the test fluid and give you a false positive for exhaust gases, even without a head gasket issue.
It's a pretty common scenario where a thorough flush can throw off subsequent tests.
Since you've pulled the heads and see no physical signs of failure, I think that strongly indicated that the initial test might have been misleading.
I'd suggest reassembling, refilling with fresh coolant, without the additives, and then re-running the block test after the engine has had a chance to warm up and circulate the new fluid.
That should give you a more accurate reading!

 

[ollie8820]

Yeah, a block test can definitely be sensitive to residual chemicals.
I wouldn't be surprised that citric acid, being an acid, could potentially react with the test fluid and give you a false positive for exhaust gases, even without a head gasket issue.
It's a pretty common scenario where a thorough flush can throw off subsequent tests.
Since you've pulled the heads and see no physical signs of failure, I think that strongly indicated that the initial test might have been misleading.
I'd suggest reassembling, refilling with fresh coolant, without the additives, and then re-running the block test after the engine has had a chance to warm up and circulate the new fluid.
That should give you a more accurate reading!

Right, if you flushed recently, it's possible the test picked up something from the acid rather than a real head gasket leak. Since you didn't see any obvious signs after pulling the heads, it might be worth rinsing the system thoroughly and retesting to be sure.

 

[Rickyboy80]

Yeah, a block test can definitely be sensitive to residual chemicals.
I wouldn't be surprised that citric acid, being an acid, could potentially react with the test fluid and give you a false positive for exhaust gases, even without a head gasket issue.
It's a pretty common scenario where a thorough flush can throw off subsequent tests.
Since you've pulled the heads and see no physical signs of failure, I think that strongly indicated that the initial test might have been misleading.
I'd suggest reassembling, refilling with fresh coolant, without the additives, and then re-running the block test after the engine has had a chance to warm up and circulate the new fluid.
That should give you a more accurate reading!

We did something similar for a friend of mine and ended up in the same boat. Make sure you clean any acid residue before redoing the test. We used a baking soda solution and ran the engine. Yes, I know many won't agree, but the idea was to clean it out. We were young and stupid and I think there are some premade solutions out there for this. After that we redid the test and got entirely different results. We also did a compression test for good measure and it was all good.