Hard start problem when engine is cold

c_anthony27

New Member
Joined
Jan 20, 2021
Messages
5
Location
seaford ny
I have a ‘91 YJ that is a project im doing with my son

had a blown engine in it according to the previous owner

we sourced a new engine and swapped it and got it up and running
it runs like a top
no noise
great oil pressure
no overheating issues etc

the only thing is that it starts up very hard when its cold
when its warm it usually fires up right away

when i did the engine swap i did put new plugs in and gapped them
brand new distributor with brand new cap and rotor
i also put a brand new fuel filter in

i can hear the fuel pump prime for a few seconds when i turn the key

looking for some other things that might be causing this that i cant think of

thank you
 
You may want to start by checking the fuel pressure at the fuel rail by the injectors. When you shut down the engine the pressure should hold for quite a while, mine dropped only 1 PSI over an hour. If there's a bad check valve in the fuel pump or possible the pressure regulator is bad the fuel may drain back to the tank. When you first turn on the key to the run position you should hear the fuel pump come on for a few seconds to prime the fuel line. That may or may not be enough to quickly start the engine.

You should be able to borrow a fuel pressure test kit free from a local auto parts store like AutoZone.

Specs for the fuel pressure and other good info can be found in the Factory Service manuals located in this forum in the resources section. YJ Resources | Jeep Wrangler YJ Forum

Cheers. Let us know.
 
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Carb?
Possible Upper and lower butterfly prob. Sounds like bottom is to far open.
‘91 is first year fuel injection HO motor

im going to try and get a fuel pressue
gauge on it this weekend and see what i get for a reading

hoping its something simple
 
Think I have it narrowed down to the check valve in the fuel pump
with it dead cold its 20 deg out also I put a clamp on the return fuel line turned the key the pump primed gauge went up to 30psi and held there didnt leak below 30psi tapped the key it fired right up on the first crank
gauge goes up to about 38lbs with it running (disconnected the clamp on the return line) let it warm up to 195 deg and it stays around 38lbs
disconnected the vacuum line on the fuel pressure regulator and the psi jumps up about 8lbs put the vacuum line back on and it returns to 38psi
so it looks to me that everything is in order except that check valve
i ordered an inline 5/16” fuel line check valve from amazon for )$10 ill install that next weekend and hopefully that cures the issue without having to drop the gas tank
 
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