A few weeks ago I posted about an old car I have that would not start. A mechanic got it running and said it was lack of compression due to the car not having been run for months. It ran fine after he fixed it. Though I had several sceptical responses to the mechanic’s theory. Well today, the trouble is back.
This time, the length of time shouldn’t be an issue. I ran the car a week ago. The gas is fresh, the battery and fuel filter is new.
1996 Nissan Sentra.
I started it this morning, it ran for a couple of seconds then stalled. Now it turns over well, but won’t start. Anyone have any theories?
Anyone with tests I can run to diagnose the problem?
Take one, some, or all of the spark plugs out and see what colour they are. They should be a fairly light, sandy brown if the engine was running right. If they have black carbon on the electrode, that indicates overfueling which could be from a blocked air filter or intake pipe.
If the plugs are oily, that could be from leaking valve stems, piston rings or the head gasket, either that or the engine’s breathing really heavy. I would check the inside of the air inlet pipe after where the breather hose fits on, and see if there’s much oil about.
Also could be worth checking inside the oil filler cap, and the header tank/radiator for any mayonnaise, which indicates head gasket trouble (although if there’s only a bit under the oil cap, and the car is only used for short journeys, this could be normal).
If all seems normal, and there are no obvious wires chafed/hanging off then I’d warm the spark plugs with a blowtorch before refitting them, and try the engine again.
If she still won’t go, you need to make sure it’s getting fuel and a spark. Check the spark by popping off a HT lead, inserting a spark plug (either taken from the engine (but watch out for fuel spraying out the hole) or a spare one if you have one) and get someone to crank the engine briefly while you hold the tip of the spark plug to earth, making sure to only touch the rubber part of the lead, away from the metal part of the spark plug and without leaning against the engine. Spark plugs fire on >10,000v and while it probably won’t kill you unless you have a weak heart or something, it’s not very nice.
You should be able to see the plug sparking brightly while the engine turns. If not, try another lead, or another spark plug. If none of the leads work, it’s either duff leads, a bad coil or no feed to the coil.
If there’s a spark, check the fuel (if none came out while checking the spark). This will depend on the engine, if it has a carburettor, simply take the air cleaner off the top of the carb and you should be able to see/smell fuel as the engine is turned (don’t put your face up to it to check). If it’s fuel injected there will be a pipe somewhere near or on the inlet manifold that releases fuel when disconnected. If your friend briefly turns the engine (or sometimes just turns the ignition on) while you point the pipe to a suitable container, you should be able to see fuel running freely.
If you’ve got fuel, spark and compression (the engine sounds normal when it turns over, and isn’t whirring around with no resistance) then you need to take it back to your mechanic, it’ll likely be a bit involved for an online diagnosis.
Likewise, if you don’t feel comfortable carrying out any of the tests I described above, take it to your mechanic. I guarantee he’ll be at least silently grateful that you didn’t leave it in pieces for him to put back together, there’s nothing more irritating (imo) than rebuilding something you didn’t strip.
Let us know how you get on.
I must have missed or forgotten the earlier post. The low compression that got better sounds pretty preposterous on the face of it, but I do recall these cars have a funny two-chain timing chain set up. I wonder if you could have a stretched chain that’s still allowing the engine to make enough compression to run but caused hard starting and no starting when combined with old gas or low fuel pressure after sitting.
If the mechanic actually had some reason to suspect a compression issue (i.e. he did a compression test) earlier, I would not be at all surprised if that it has now gotten worse.
I once had an old diesel Volvo. It could be a bit hard to start. I did an oil flush to clean out the system. Apparently the gunk was the only thing keeping the compression at a tolerable level. The car went from hard to start to pretty much unstartable.
the thin layer of oil on the cylinder wall/rings does contribute to cylinder sealing. I’ve seen older cars (carbureted) where there was a massive over-rich condition that caused excess fuel to wash the oil off of the cylinder walls, dropping compression massively. a squirt of oil in each cylinder took care of that.
This is the explanation the mechanic gave me. that is, the layer of oil in the cylinder wall drained off and compression suffered. As other mechanics have said, it seems unlikely. And the fuel was old and hence had lost octane. Combined the engine wouldn’t start.
I tried it again today after a cold night and it started. Obviously there is a problem, but not insurmountable.
I am puzzled by the symtom that it starts for a second or two, then dies and won’t start at all. I wonder if it might be a bad fuel pump? Can a fuel pump slowly drain and start to cavitate? Would that cause such a problem?
it’s not unlikely if what you say here is true; if the gas had gone so stale that it was hard to ignite then it surely could wash down the cylinder walls. if you’re trying to start the thing with stale gas, and it’s not igniting, the injectors are just going to keep squirting more and more fuel into the cylinder.