Im baffled

i have a 1998 GMC sierra pick-up that will not start when the weather gets cool and damp.Just a little dew on the ground can do it,but i can spray it with the hose all day long and starts right up.Ive replaced many sensors,fuel pump etc…HELP !!!

You need to see if the plugs are getting spark when this trouble happens.

Same thing happened to me when I had bad spark plug wires…

plenty of spark…new plugs and wires…plenty of fuel pressure to…it acts kinda like timings off…but soon as sun comes out it starts up and runs great…i cleaned all elec. connections and used dielectric grease…im curious if an oxygen sensor could be at fault ?

thanks for your input,gonna start with new dist.cap as i didnt replace that…i`ll let you know … im open to all suggestions so anyone else with info send it on…

To run, the engine needs; compression, fuel and spark.

I can’t see any way that humidity would affect the compression so rule that out.

The simplest test is the one I would do next. Loosen the snorkel at the throttle body. While someone cranks the engine, spray a bit of starting spray (ether) into the intake. If it spits and sputters or runs momentarily, you know you have a fuel issue. You said the fuel pressure was good so the next test would be to use a noid light or back probe the injectors to see if they are being driven by the ECM.

If the starting spray has no effect, then I’d check for spark. They sell some spark checkers that go inline with the plug that will “light up” if the plug fires. It might still be a weak spark though, so I prefer to go old school by removing a plug and holding it against the engine with a stick to see if it sparks while someone cranks the engine. You can see the quality of the spark that way.

You can better simulate the high humidity conditions by using a misting bottle in various areas to try and isolate the susceptible areas. The ignition system can be checked this way at night. Any electrical shortcuts are easily spotted at night.

i have spark and fuel.i replaced the spider valve inside throttle body(sends spark to injectors). i even replaced the pcm,the ignition control module,camshaft pos. sensor, maf sensor,crank sensor, throttle pos sensor,knock sensor,fuel pump,plugs and wires,rotor,map sensor and a few more i cant remember…it acts like timings bad when this happens,but otherwise cranks right up and runs great…im stumped…

Going back to bad spark plug wires and old dist. cap. The timing acts “bad” because the current to make the spark plug fire is jumping over the wires and causing the wrong plug(s) to fire at the wrong time. In effect it is a timing issue. The moisture changes the “conductivity” of the wires and that’s why it is fine when it is dry and whacko when moist.

From reading all you’ve done on this post and the other one I’m more convinced that the “new” wires aren’t doing a good job and the cap is likely bad too. Put the new cap on first and see what happens, but I think you would do well to replace the wires too. Perhaps you could look for a set of “marine” plug wires for a similar V8 engine. They are made to perform in moist enviornments.