Hard starting Jeep

Factual Questions
1

My girlfriend’s 88 jeep cherokee has never failed to start, but it always needs to be turned over 15 odd times before it’ll fire.

I’m thinking something simple and easily replaceable by yours truly will solve this little problem.

fuel pump? pcv valve? fuel/air filter?

This model is equipped with “Throttle Body Injection”, which according to “the book”, is “only under pressure when the fuel pump is operating”.

It seems to this layman that the fuel pump would be to blame, but I’m usually wrong about these things.

I can usually easily replace parts if I know what’s broken, as I just fixed (read: had to replace) her brakes - I just need a little advice with these diagnostics before I waste the money to replace all of the above when one or none might suffice.

Thanks

2

Lots of possibilities. Make sure it’s in good tune (spark plugs, plug wires, etc.).

First guess is that the fuel system is not maintaining pressure in the lines, so that each time it’s started (after sitting for some time) it’s necessary for the pump to re-pressurize the system. The check valve responsible for this is an integral part of the fuel pump. To test for this possibility, do a complete pressure test of the fuel system (recommended approach) or replace the fuel pump and see if that fixes it (mighty expensive “test” if it doesn’t fix it).

3

First thing to change is the spark plugs.

What kind of maintenance is she doing on it? How often is the fuel filter changed?

4

My vote is for the check valve too. After it sits a while, the check valve (or ball), could be relaxing and letting fuel back into the tank, this will cause hard starts as the fuel has to pump fuel back into the lines which could cause “15 odd times” of starting.

First thing, get a fuel pressure gauge. Best way to find out would be to get the car up and running, shut it off and measure the pressure right away and record the PSI. Next, run the car again, shut it off, let is sit for a few hours, take the measurement again. If there is a huge difference is PSI, it’s your check valve leaking.

FYI, you would connect the fuel pressure gauge to a fitting found on the fuel lines near the throttle body, in case you’re wondering.

5

Thanks for the replies.

The plugs are new, but the fuel filter hasn’t been changed in years.

Is it possible that this hard starting is normal for a “throttle body injection” system which is “only under pressure when the fuel pump is operating”?

I’ll get a fuel pressure guage but and check it out but it almost sounds like it loses pressure by design. ?

6

Throttle body injection as opposed to multipoint (port) injection is not relevant here. In either case, the injectors are electrically controlled, and only flow fuel when signalled to by the ECU (computer).

I don’t know where this came from–“only under pressure when the fuel pump is operating”–but my Mitchell manual described a check valve in the fuel pump. Every electronic fuel injection system. I’ve seen–on any kind of car–maintains residual pressure in the lines even when shut off. This residual pressure may be somewhat less than full operating pressure, and it may only have FULL pressure when the pump is operating. Check a manual for the applicable pressure specifications.

Hard starting as described is not normal for any car, ESPECIALLY any modern car with gasoline fuel injection (of whatever type).

7

Oh, and change the fuel filter. While I doubt it’s related to the starting problem, it’s not totally out of the question, and replacing it will prevent future problems. A partially clogged fuel filter can overstress the fuel pump.

8

The quote above came from the Haynes Repair Manual, Jeep Cherokee 84-2000, Chapter 4 section 2.

This reads to me like there should be no residual fuel pressure when the engine is turned off.

But I just discovered this is moot anyway. Her car has an inline six which has Multi-Point Injection, which should have constant fuel pressure of 39 to 50 psi.

I appreciate the help. I think I have enough info to fix this now.

9

Also just wanted to throw in that my Haynes repair manual has instructions for a MPI fuel pressure leak down test, “If an abnormally long cracking-period is required to restart a hot engine after the vehicle has been shut down for a short period of time, the fuel pressure may be leaking past the fuel pressure regulator or the check valve in the outlet end of the fuel pump”.

Bingo. Exactly what you guys said. It’s easy when you know what to search for.

Thanks again.

10

“but the fuel filter hasn’t been changed in years.”

You usually change them every 20,000 miles, so if you got one, change it.