'93 Wrangler YJ 4.0 doesn't start: turns over, good spark, 40 psi at fuel rail

Wyvern_64

New Member
Joined
Jan 7, 2023
Messages
2
Location
Alameda, CA
Hi all,

I have a '93 Wrangler YJ 4.0L (automatic transmission), and I'd like help diagnosing it. Here's what I've observed so far:
  • I started it recently and drove it a short distance. About an hour later, it would turn over but not start.
  • It turns over just fine every time, so the starter system seems good. 12.5V at the battery
  • I think it has good spark - I disconnected the spark plug wires from 2 of the spark plugs and confirmed spark on both of them (they were the easiest to get to), and I also disconnected the spark plug wire from the ignition coil at the distributor and confirmed good spark there.
  • With a fuel pressure tester connected to the Schrader valve on the fuel rail, I measured about 40 psi while cranking. In my online research it seems like this is about right?
Any ideas for what to look into or try next? I'm at a loss, and in all of my online research people's issues have been one of those 3 things above. The only thing I suspect is the crankshaft position sensor (leading to bad spark timing), but I thought it wouldn't even spark at all if that was bad.

Thanks in advance!
 

Flyer58

YJ Enthusiast
Joined
Apr 12, 2020
Messages
723
Location
Magnolia, Texas
Are the spark plugs wet after cranking? The injectors may have pressure but no signal to open. A shot of starter fluid or engine fogger may get the engine to fire off and if it immediately quits then you know it's not getting fuel past the injectors.

There's probably a way to test each signal and also check the injector coil ohm for continuity while you're there.
 

Piratesailor

Member
Joined
Sep 13, 2022
Messages
31
Location
Texas Coast
Sometimes it’s simple as a clogged filter. Sound like no gas to the engine and it’s all as it should be. Sometime sediment can be picked up. Your issue could be in the fuel tank
 

JKP

Member
Joined
Feb 20, 2021
Messages
37
Location
Warner Robins GA
If the IAC is stuck closed it won't start. As you are cranking, slightly press the gas pedal. If it starts and runs as long as you press the pedal, but dies if you realease it, the IAC is stuck closed.

Spray some starting fluid down the throttle body, if it runs on that, you have a fuel problem. But 40 PSI should be plenty.

Long shot, but you can rotate the crank to TDC on cyl 1 and verify that the distributor rotor is pointing to the cyl 1 terminal. If not, timing chain has jumped.
 
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machoheadgames

Certified YJ Researcher
Supporting Member
Joined
May 19, 2020
Messages
1,512
Location
Dallas, TX
Yup, IAC would be what I’d look into. If your spark and fuel are good, only thing left is air. If the IAC is stuck and not retracting to give the motor air, then it won’t start. Holding the throttle open by pressing the pedal while cranking would narrow it down. If it starts while holding pedal, then you need a new IAC. If that doesn’t fix it, then you have a spark or fuel issue which was incorrectly diagnosed when deciding that spark and fuel were good.

Definitely check out the IAC.

Fuel pressure should be 31 psi at idle with the fuel pressure regulator hose connected and 39 psi with it disconnected (simulates high load conditions - regulator is vacuum operated and ups the pressure when you’re on the gas hard).