Hard start ups

[Motorhead X]

What would cause intermittent hard starting? My MJ usually starts right up, but recently it is taking too long, more and more often. I’ve checked the vacuum system, replaced the MAP sensor (idles a little better now), O2 sensor was replaced about 4 to 5 years ago. I’m considering replacing the cat converter because it has stumbled a couple times on startup in the last few months. It’s around 10 years old and looks okay. No other obvious cat clogging signs like overheating or rattling. Runs fairly well otherwise, maybe getting a bit worse gas mileage. Plugs are only at 20K. If I do the cat, I’ll have to take it to a shop because I don’t weld. Thanks.

[pizzaman09]

By hard starting, are you saying that it doesn't start or is taking more cranking than it has in the past?  I see you have an HO 4.0L so that I'm a bit less familiar with.  I'd be inclined to think that a sensor like the crank position sensor is more likely to be an issue.  If you are suspicious of vacuum leaks, make sure the intake/exhaust manifold bolts are tight.

[Motorhead X]

More cranking than normal. I’ll check those manifold bolts. I changed the exhaust manifold a few years ago, so could be the culprit and I am suspicious of vacuum leaks. If no luck, I’ll give that CPS a shot. Thanks.

[89 MJ]

Does it seem to do it more when it’s up to temp or when it’s shut off? Or is it regardless of the truck’s temperature?

[Motorhead X]

53 minutes ago, 89 MJ said:

Does it seem to do it more when it’s up to temp or when it’s shut off? Or is it regardless of the truck’s temperature?

More often when it’s up to temp. But still can be regardless of temp. I let it sit for a few days and it started right up. But then I start driving it more and it seems to act up. Seems obvious to me now I should replace the temp sensor. Thank you!

[89 MJ]

I was going to suggest it was heat soak since it seems to do it more when warm. The fuel is boiling in the rail and it should have a heat shield. I can’t remember if HOs have a return style fuel system or not. 

[ghetdjc320]

HO's do have a return style fuel rail until the obd2 era. Try this, prime your fuel pump a few times before starting by turning the key to the "on" position and keeping it there for a few seconds. do this maybe 3-5 times before starting it, then try cranking it and starting. Let us know the results.

[Motorhead X]

Update… I swapped the temp sensor and no change. Hard startups are becoming more frequent, regardless of engine temp. But it’s the worst when the engine is just kinda warm after sitting for a couple of hours. Starts a bit better when cold or at operating temp. I’m considering changing the CPS. Could that cause difficult starting, or is that more of a "it works or doesn’t" type of thing? Do MJ’s ever get vapor lock?

 

Side note - I heard the strangest screeching sound coming from under the driver’s side suspension area while going 70mph down the highway today. My first reaction was Oh Crap WTH! Turns out it was just a long thin stick that was howling. Whew. 

[89 MJ]

3 minutes ago, Motorhead X said:

Do MJ’s ever get vapor lock?

Yes. Make sure your heat shield between the injectors and intake/exhaust manifolds is there. Mine was in rough shape, so I tossed it and have regretted it since. 

 

6 minutes ago, Motorhead X said:

I’m considering changing the CPS. Could that cause difficult starting, or is that more of a "it works or doesn’t" type of thing?

Before replacing it, you should test it first. But it can go bad over time. Here's the testing procedure: