power loss during acceleration

[dukeboy01]

I have a problem with my 96 cherokee (130k, 4.0 I6) that seems to stump a lot of gearheads that I know, I'll tell you as many details as needed, hopefully someone has had a similar experience and can help:

When accelerating to cruising speed, ONLY the first time after startup, the engine suddenly bogs in rpm and loses power somewhere between 15-35 mph (sorry, no tach). This power lost can last anywhere from 1 second to 4 or 5 seconds, and only seems to get worse if I press on the gas more. When it does pick back up, it REALLY picks up and I can feel a very noticeable difference in power. As I mentioned, it only does it within seconds of starting the vehicle, and accelerating the first time after startup.

It is fairly sparatic on doing this, not every time, sometimes it can go days without doing it, then sometimes 3 times in a row one day (upon consecutive starts).

Seems to do it whether engine is hot or cold.

Air filter is good.

Fuel filter replaced about 20k ago.

Seems to be independent of fuel level in tank.

Tranny fluid level and color is good, flushed about 30k ago.

New TPS sensor about 35k ago

Any help would be great.

[AlohaBra]

I believe this is normal, after startup the ignition is kept retarded to warm up the cats with a rich mixture. My 97XJ does this if I accelerate too much when starting off and make a popping noise at the air cleaner. My 01 Mazda Tribute does this too when starting off even on a hot day. I have own the 97 xj since new. I just take it easy for the first 30 seconds after startup (which is how long it is retarded, I think...could not find actual number about this).

[Saudade]

OK, so clarify your problem, if you sit and idle for 30 seconds or so, it does not happen? Are the "consecutive starts" when the engine is cold, or does it happen when the engine is still warm after use?

I would check the fuel pressure. Maybe it needs a few more seconds to build up.

Also, IMO, it's bad practice to routinely take off immediately after starting, especially from a cold engine. You should give a cold engine a brief warmup to get the oil pressure up and ensure it's circulating or drive around slowly first.

[dukeboy01]

Thanks for the replies so far:

---- "My 97XJ does this if I accelerate too much when starting off and make a popping noise at the air cleaner"---

The popping noise definitely sounds familiar, I hear it right when the engine starts to pick back up and take off again...Are you sure it is from the air cleaner? I thought maybe it was just drivetrain lash or something.

I can't say for sure so far (I'll pay more attention now) that it only does this within a very short time after starting, it may have done it a few times before even if I have let it sit at idle for a minute or two before driving.

Also it seems to do it more often when the engine is hot (operating temp). Typically in colder weather upon first start I always allow the engine to warm for several minutes.

I forgot to mention, no check engine light or codes.

[dukeboy01]

Oh also what should correct fuel pressure read?

[Mac53]

Hi,

When you start the jeep does it take 5 to 10 sec. to start? I believe fuel pressure should be 32-34. I had the same problem. pressure was 21, slow starting and no acelleration, or slow acelleration. Epecially when dropping down in speed from 65 to 30 and trying to get back up to speed I would have nohing. I replaced the o2 sensor, no help. Finially talked with some guys and lack of pressure could only mean the fuel pump. BINGO!!!!, I replaced it last week and she runs like a champ. I think a garage will charge you between $600 and a grand for a change out. It wasn't really that bad to do, I had to drop the tank, Chiltons says you don't have to, but trying to get the retaining ring on is a bitch with the tank up, took about 6 hours and some beer. Next time probly 3 hours............good luck.

Mac

[Saudade]

Check out:

http://www.autozone.com/servlet/UiBr...3d801528b9.jsp

[surf1324]

Did you ever figure this problem out? Mine does the exact same thing that yours does... with a knocking kind of noise or backfire coming from the engine. I have changed the tps, iac valve, map sensor, pulled and cleaned the injectors, cleaned the intake manifold and it still does it... someone told me the fuel filter but if that was the case, it would hesitate a lot more than just once after starting. Help?

[dukeboy01]

To answer your question, it still remains a mystery to this day.
I've learned to live with it and it has not gotten any worse, though it is quite predictable.

I'm going to have to agree with the guy who said...

"I believe this is normal, after startup the ignition is kept retarded to warm up the cats with a rich mixture. My 97XJ does this if I accelerate too much when starting off and make a popping noise at the air cleaner."

I think that a lot of things can be done to try and diagnose the problem (and a lot of money spent doing it) when in the end it might just be "a jeep thing."

I just let er run about 30 seconds (does seem to be the magic number) before I take off and its fine. Now at 140k and the vehicle runs flawlessly otherwise.

Good luck!