Engine goes flat at 4,200 RPM

[HarleyMick]

I have a 99 Jeep Wrangler Sahara and I recently changed the engine to a 4.7L Stroker. It is balanced with Chevy valves and springs and a CompCam with a basic operating range of 800 - 4800 RPM. It also has a header with low restriction Flowmaster exhaust, a 62 mm throttle body and 24 lb. Mustang fuel injectors.

Here's the problem: At about 4,200 RPM it sputters and looses most of its power until I let off the gas. It does this at any rate of acceleration and in any gear. I used a Hypertech programmer to add 500 RPMs with 91 octane gas. The camshaft pretty much demands the timing to be set at 15 degrees BTDC wich forces me to use 95 octane to keep the valves quiet. I checked the fuel pressure and it levels out at 42 psi, which is less than the 43.5 minimum required by the injectors.

What do you think? Am I running out of fuel pressure due to a week fuel pump or is it a ECM/PCM issue? I'm retired and hate to just throw more money at it. There is a company I could send the computer to with all the info I gave you and a little more to reprogram it specifically for my setup. But what if it's the fuel pump instead? Any ideas or advice would be greatly appreciated. Anyone?

 

[Nimbley]

Sounds like you’re running out of fuel. Did you check fuel presure while driving? It’s probably dropping off as fuel consumption goes up. If it’s not a restriction on the delivery like a kink, sharp bend in the fuel line, plugged filter, etc. then it’s a bad pump.

 

[TJ Hunnicutt]

You could certainly be running into a lean condition at those RPMs with less-than-minimum-suggested fuel pressure.

Have you considered reaching out to the people you got the engine from? More often than not, they have people who can help diagnose new-install issues.

 

[HarleyMick]

Excellent ideas, both of you. Very much appreciated. I was leaning towards a performance fuel pump as the engine manufacturer says it is totally compatible with the ECM/PCM on performance settings. I just wanted concurrence from a fellow Jeepster or two!

 

[jtf440]

I'm fairly sure it would be easy enough to retrofit a bigger fuel pump in the place of the factory one, but I'm not sure what your experience on fuel systems is. I bet others on here could help you.

 

[Flivver250]

When you fix it, give us an update. Love to hear about your stroker.

 

[HarleyMick]

When you fix it, give us an update. Love to hear about your stroker.

I'll have my brother take a picture of it pulling the front wheels. I figured with 4.88 gears I'd be stuck at no more than 70 MPH on the interstate. But 5th gear on the AX15 trans has so much overdrive 70 MPH is only 2700 RPM.

 

[TJ Hunnicutt]

Wow, that's a clean engine compartment! Very nice.

As a side note, I would consider going back to the factory air intake system. It has been shown (over and over) that the factory system, with a good quality filter, far surpases the air cleaning ability of the "cold air intake" systems with no degradation in power. The 4.0 just does not require it nor benefit from it.
I'm fairly certain that the RamAir system that the PO installed on my 2001 was the major cause of the engine's demise. You've, no doubt, invested a good amount in your stroker and I would hate to see it destroyed by grit for no gain.

 

[HarleyMick]

Wow, that's a clean engine compartment! Very nice.

As a side note, I would consider going back to the factory air intake system. It has been shown (over and over) that the factory system, with a good quality filter, far surpases the air cleaning ability of the "cold air intake" systems with no degradation in power. The 4.0 just does not require it nor benefit from it.
I'm fairly certain that the RamAir system that the PO installed on my 2001 was the major cause of the engine's demise. You've, no doubt, invested a good amount in your stroker and I would hate to see it destroyed by grit for no gain.

I appreciate what you are saying and generally agree. However, the company that built the 4.7 long block with an aggressive camshaft, stronger crankshaft and larger Chevy valves with stronger springs recommended the cold air intake to compensate for those changes plus the header, 62 mm throttle body and 24 lb. Mustang fuel injectors. The engine is approaching 300 hp and requires the extra air.

I do keep the filter clean and oiled as is required and this TJ will never see off roading again or even rain for that matter unless it is a total surprise. I am thoroughly preparing it for car shows and I am currently working very hard with a chisel, sander and other tools as I remove undercoating and add Hot Rod black semi-gloss paint to the frame, axle housing, etc. to make my 20 yr old Jeep look new in all areas.

Up until a year ago we would be on the same page. Thanks for the compliment. I could not find a polished stainless steel valve cover or even an aluminum one without the 4.0 on it. I stripped and sanded all of the black paint off, polished the hell out of it and added a couple of clear coats to protect it. I'm pretty obsessive and possibly a little anal retentive.

 

[pagrey]

Call Golen, you might not need an aftermarket fuel pump. This is their business, they probably know the answer.

 

[whitrzac]

99% most likely an issue with the fuel system. You might not need a larger pump, just a new one with a new/clean fuel sock and filter.

If that doesn't take care of it, things get a little more complex.

 

[HarleyMick]

Thanks, guys. I'm hoping the fuel pressure regulator is stopping it short of the 43.5 minimum PSI required is the problem. My biggest worry is that the ECM/PCM needs to be sent somewhere to be reprogrammed for my needs.

 

[TJ Hunnicutt]

I appreciate what you are saying and generally agree. However, the company that built the 4.7 long block with an aggressive camshaft, stronger crankshaft and larger Chevy valves with stronger springs recommended the cold air intake to compensate for those changes plus the header, 62 mm throttle body and 24 lb. Mustang fuel injectors. The engine is approaching 300 hp and requires the extra air.

I do keep the filter clean and oiled as is required and this TJ will never see off roading again or even rain for that matter unless it is a total surprise. I am thoroughly preparing it for car shows and I am currently working very hard with a chisel, sander and other tools as I remove undercoating and add Hot Rod black semi-gloss paint to the frame, axle housing, etc. to make my 20 yr old Jeep look new in all areas.

Up until a year ago we would be on the same page. Thanks for the compliment. I could not find a polished stainless steel valve cover or even an aluminum one without the 4.0 on it. I stripped and sanded all of the black paint off, polished the hell out of it and added a couple of clear coats to protect it. I'm pretty obsessive and possibly a little anal retentive.

Wow, a show Jeep. How cool! I'd say that's a rare thing in the TJ world.

 

[HarleyMick]

Wow, a show Jeep. How cool! I'd say that's a rare thing in the TJ world.

You know, after 20 years I think my Jeep deserves a thorough spruce up and by their criteria is exposed to a crowd not normally associated with Jeeps of any kind. I hope to win a trophy or two showing people a glimpse of what our world is all about by making it look like it's virtually brand new. They will also see the full functionality of the 6 inch lift, heavy suspension, 35" X 12" tires and of course the power of a vehicle designed to be fun on and off the road.

You know what I would prefer? A Jeep show where we might wash and shine it, but mostly give a bunch of Jeepers a chance to mingle, trade stories and compare mods without judgement except on design for things like trail riding, rock climbing, mud running and maybe pulling power.

There are a lot of Jeeps in the upper South Carolina area and I see some cool looking rides on parking lots or in passing on the road. Yeah, we do the Jeep wave but keep going. I wish we could all meet and just compare what we do and share ideas. BYOB

 

[pagrey]

Some Jeep clubs have meetings at the local diner that don't involve trails if that's your thing. A few people even enjoy the meetings more than the trail runs.

 

[HarleyMick]

Changing to a high performance fuel pump did not help, it still cuts out at 4,500 RPM. It has to be the rev limiter in the ECM. Any ideas on where to send it without getting ripped off or robbed to have the programming changed?