'91 4.0L motor not starting

FWIW, the FSM specs no more than a 20psi drop in fuel pressure after being shut down for 30 minutes. Thus, if you have the normal 31psi idle fuel pressure, it should be at least 11psi 30 minutes after shutting down.

(My experience is that it raises to 35-38psi if the motor is hot due to expansion from heat soak, then slowly bleeds away after several hours).

If you have a leakdown problem, and you don't see an external leak, suspect:

(1) The in-tank hose section
(2) The check valve (at the outlet end of the in-tank pump)
(3) The fuel pressure regulator

For what it's worth...

I do have more trouble with hard starts in "transition seasons" when they're switching blends. Seems as if at the seasonal switch, the lower vapor pressure blends boil in the rail when shut down. This makes it hard to restart because there's vapor left in the rail that hasn't bled out. It'll "catch" and stutter for a few revs, then clear up and idle right.

You have some pretty cold temps up there right now...it will be difficult to get enough fuel vapor into a cold motor to get a start without it pooling and flooding.

Thanks for all the answers. The flooding is definitely a possibility. I actually thought it acted like flooding, but I didn't know that was possible with the MFI system.

Cycling the key on and off doesn't make it start faster, and I can hear the fuel pump motor running. But a pressure check will tell more about the state of the fuel delivery system. I'll check the rail pressure with the vacuum line connected to the regulator, and with it removed. The FSM says it should be 31 with vac connected, and 39 disconnected. I'll also check for bleed down of pressure.

The Bosch injectors are only a few years old, so I doubt they are leaking into the cylinders, and they are definitely not leaking through the o-rings.

As for the cold, I've been running this Jeep for about 11 winters up here with some much colder temps than we have now, and there have been no problems (other than the longer cranking time that I get all year round).

I'm going to do one thing at a time, and test the results of each action. Then we'll begin to know what caused this problem.

The past three times I've tried, it has started right up, but not as quickly as when I first installed the motor. Even though it's starting now, I want to get on top of this ... just to be sure I won't get stranded up on some mountain logging road in a snowstorm. This will be a good time to do a thorough check of all the sensors and fuel delivery.

Thanks!

Problem Solved.

First thing I did was clean the Manifold Air Temperature (MAT) sensor. It was crudded up with black gunk. Cleaned it, reinstalled it, and the motor fired up much faster.

Next, I removed and cleaned the throttle body, the TPS, and the IAC valve, and put it all together with a new TB gasket. It starts in less than 1 second now. Runs smooth as silk. The IAC was *all* gunked up with thick black tar. I scraped off big chunks of it with a popsicle stick before using the TB cleaner. When I put in a new motor 11 years ago, I cleaned out the intake manifold as best I could, but didn't remove the TB and components. I didn't know anything about fuel injected systems and sensors back then. So I don't think the IAC had been cleaned in 25 years.

I tried to check the fuel pressure, but the crap loaner unit at O'Reilly's leaked where the hose was swaged to meter. So I never got a reading on it. I'll check that in the Spring.

The main thing was cleaning the MAT sensor. It definitely fired up faster after that.

Only $2.00 for the TB gasket, plus a little TB cleaner I had on hand anyway.

Thanks to everyone for your help.

Great. I was shocked at how much grief a dirty MAT sensor can cause. Only way the MAT and IAC can get fouled like that would be accumulated blow by routed (OEM) to the intake. On older Renix rigs the EGR would add to that problem.

Ecomike said:

Only way the MAT and IAC can get fouled like that would be accumulated blow by routed (OEM) to the intake.

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Yep ... I've got a little oil in the air cleaner box and could see a little oil in the intake manifold. The motor has about 67k miles on it, and I replaced the CCV hoses when I installed it. I guess I'll have to clean the CCV vacuum hoses and the "outlet" at the back of the valve cover. At one point, I was thinking of making an oil catch can like on the old MadXJ site.

I got ride of that problem finally, once I replaced every single source of a vacuum leak on my 87, including the entire CCV hose assy and the valve cover gasket and grommets. Seems the tiniest vacuum seal leaks on the CC cover, grommets, gasket and CCV lines, can make the crank case gasses vent in the wrong direction at times (idle, acceleration, decel?) and the rear CCV line and orifice clog up enough to where they do not move enough air to capure the blow by so it heads for air filter in default.

montanaman said:

Yep ... I've got a little oil in the air cleaner box and could see a little oil in the intake manifold. The motor has about 67k miles on it, and I replaced the CCV hoses when I installed it. I guess I'll have to clean the CCV vacuum hoses and the "outlet" at the back of the valve cover. At one point, I was thinking of making an oil catch can like on the old MadXJ site.

Click to expand...