Starting Stumble

[Anime]

A few weeks ago my X had some intermittent stumbling on cold start in the morning, once every few days it would start, but immediately after the RPMs went up, instead of dropping to fast idle speed, they dropped to ~500 RPM and it would stall. Turn the key again and it starts fine, works fantastic the rest of the day. This gradually increased in frequency until it became a daily occurance.

Because the fuel pump was making noise that was audible inside the vehicle with the engine running, I've replaced the entire fuel pump assembly (NAPA) thinking it was the check valve or just a worn out old pump that was on its way out, and for two days or so after that, there were no problems. (Seemingly unrelated, the battery died during the fuel pump install, weird) Now the same stumble on cold start in the morning is back, but it doesn't stall out. The RPMs still drop, but pick up after 1-2 sec. and go right to fast idle speed. The new fuel pump is still quite audible from outside the vehicle (window down), but gets quiet when driving, even just getting rolling in 1st gear.

I'm looking for suggestions, at least hoping to reduce the cost of throwing a FPR, IAC, TPS, CPS, etc. at it until it's fixed. I'm considering replacing the Fuel Pressure Regulator, but theres no fuel in the vacuum line connected to it or the vacuum tree. However, I do have zilch for fuel pressure at the rail after it has sat overnight. The fuel pump does cycle on when the key is turned, but even turning it on and off a few times before starting doesn't seem to make any difference, the stumble still happens. This definitely seems fuel related since a new pump made a difference, going from a stumbling then stalling problem to just stumbling. There are no other problems, idle speed and performance are fine the rest of the time. I noticed in a few other threads some others are having this issue, so help us out! Thanks in advance.

 

[TPLYNCH]

Try tightening the intake manifold bolts.

 

[Anime]

This was done before, but yesterday I re-checked the torque on both the upper and lower intake manifold bolts, and they are torqued to spec. It didn't make any difference in starting this morning, the engine actually stubled twice, once and even again after tapping the gas.

The fuel filter was changed not long ago, so I don't think thats the problem, also because theres no problem with throttle response or fuel delivery any other time.

 

[MrShorty]

What is the fuel pressure at the rail?
Have you cleaned the IAC?

 

[Anime]

Fuel pressure at the rail is zero in the morning after sitting overnight, but goes right up to normal pressure when running and the rest of the day.

I haven't removed or cleaned the IAC since there is no problem at idle, its perfectly smooth like a new engine. The only problem is this stumble when starting cold first thing in the morning. Seems those with IAC issues have had problems with the idle and general performance of the engine when driving, not when first starting up. I'm willing to entertain the idea it could be IAC related, though. I'm just hoping someone else had this same issue and found out what was causing it.

 

[MrShorty]

There's a TSB for the early 2.9's regarding stalling on cold startups and/or hesitation when running cold. One possible cause is sludge build up in the throttle body and/or upper intake, and/or IAC. I realize a '92 4.0 is not exactly like an '86-'87 2.9, but the throttle body and upper intake and IAC are similar. As easy as it is to clean, I'd go ahead and do it.

 

[ld50]

Mine`s starting doing this very thing, I actually suspect the IAC because I swear it was howling the other day. Although it hasn`t howled since, every other startup seems to result in a stall. I`m gunna just clean it tomorrow and see.