Another high RPM power loss

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Another high RPM power loss

Postby melgav » December 14th, 2013, 2:26 pm

Hi guys,

I have been lurking for some time doing research on buying a crate engine. Being in Australia this was a major investment especially since I purchased the engine from the US. Now that it is installed I have a problem with the engines and I am at a loss as to what the problem is. I have read several posts with similar problems but have yet to find an answer, so I need help into what to look for. I am not a mechanic but my mechanical ability is not too bad.

My Jeep is a 1999 automatic Cherokee Sport with the following modifications, including parts installed to find the problem:

4.6 Stroker
Mild cam (all I know is that it is comp cam)
24lb injectors
Spectre air filter/snorkel (not cowl)
63mm bored throttle body
2.5” high flow cat back exhaust
New fuel module
All new sensors except oxy sensors
New ignition system including dizzy, pulse ring, plugs, leads and coil
New radiator including separate transmission cooler with fan
Water pump, thermostat and housing

The problem is a high rev power loss (sound familiar). Around 4,000 to 4,200rpm the engine looses power and struggles to rev to redline. Here it eventually changes gear and runs really rough for a little bit, then you can feel the power build back up until you hit the 4,000 mark again.

The problem does not happen when the engine is cold. When cold it will run really well and rev out completely. Also it seems to only do it under load.

I have tested the engine compression and this checked out good.

I have tested the fuel pressure with the old fuel module and at idle running it was a 49psi. When the engine suffered it’s problem it did drop to around 39-40psi. While running rough it ran at around 46psi.

It has been on two dynos, the first was a waste of time. They claimed valve float, which I am pretty sure it isn’t, and the engine builder is confident of this too.

The second dyno was done for free by another company so i don’t have any reports, but he believed that it is dumping massive amounts of fuel when in open loop and WOT and causing the engine to run way too rich.

I have tried a map adjuster recently as well. I had it dialed back to 4 volts and the problem still persisted. I have even tried the stock 22.5lb injectors and the problem still persisted.

I am now posting this to get some help from you guys as it is really giving me the s%$^s!! :frustrated:

Any help would be appreciated!
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Re: Another high RPM power loss

Postby SilverXJ » December 14th, 2013, 5:38 pm

melgav wrote:I have tested the fuel pressure with the old fuel module and at idle running it was a 49psi. When the engine suffered it’s problem it did drop to around 39-40psi. While running rough it ran at around 46psi.

What about with the new module? 39-40PSI is out of spec.

Do you have a wideband AFR gauge?

What camshaft and valve springs does the engine have? Valve float can be an issue.
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Re: Another high RPM power loss

Postby melgav » December 14th, 2013, 6:24 pm

If the fuel pressure is out of spec that is interesting, I was not sure if it changes under full throttle. I must admit I haven't tested the fuel pressure with the new module, I will try and do that in the next day or so. The jeep needed a new fuel pump/module anyhow as it was suffering the 1/4 tank left hand turn problem and the gauge was not working properly. So this was fixed. I have also been led to believe that the stock pump is sufficient for a stroker?

I have installed a wideband gauge. Under normal conditions it runs nicely around the 14.7.

At full noise it does seem to run very rich, around 12.5 and more. When it changes gear it and runs rough the air/fuel is still around 13.5.

I honestly don't know what cam shaft is in it and I have been assured that it has springs that can cope with the revs (it is not a Titan engine, it is a Golen engine). I was under the impression that with valve float it only occurs at a certain RPM. My issue has some lingering effect, if that makes sense.

Thanks SilverXJ
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Re: Another high RPM power loss

Postby melgav » December 14th, 2013, 8:36 pm

Just an update, retested the fuel pressure with the new fuel module.

It runs at 49psi, maybe drops to 48psi when the problem occurs, otherwise fuel pressure seems OK.
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Re: Another high RPM power loss

Postby Cheromaniac » December 14th, 2013, 11:02 pm

melgav wrote:I have also been led to believe that the stock pump is sufficient for a stroker?

I have installed a wideband gauge. Under normal conditions it runs nicely around the 14.7.

At full noise it does seem to run very rich, around 12.5 and more. When it changes gear it and runs rough the air/fuel is still around 13.5.

I honestly don't know what cam shaft is in it and I have been assured that it has springs that can cope with the revs (it is not a Titan engine, it is a Golen engine). I was under the impression that with valve float it only occurs at a certain RPM. My issue has some lingering effect, if that makes sense.

Thanks SilverXJ


G'day. :)
Firstly the stock fuel pump is certainly sufficient to fuel a stroker so that's not an issue.
Secondly, you mentioned that you have a Golen engine so it'll have the Compcams 68-235-4 cam and it should have the correct valve springs (or so you'd hope anyway).
Thirdly, valve float occurs at higher rpm if the valve springs are too weak to close the valves quickly enough and prevent them from hanging open. I wouldn't rule that possibility out in your case.
Fourthly, an A:F ratio of 12.5 under WOT is acceptable so there's no problem there.
Finally, you might want to check if your distributor is correctly indexed 'cause that could be the root of the problem.
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Re: Another high RPM power loss

Postby atias » December 15th, 2013, 2:37 am

check the lifters preload also , if the preload is too high (too long pushrods), the lifters can get pumped up with oil in high revs and it wont let the valves close completely.
its a very similar symptom to valve float issue.



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Re: Another high RPM power loss

Postby melgav » December 16th, 2013, 2:55 am

Thanks for the help and suggestions so far.

What I don't get is why does it do it only when hot and not cold? Also it does it under load, so if it is parked and revved it doesn't do it, and it tends not to do it when you go full throttle down hill (also little load). I have noticed if you put the transmission in the 1-2 and gently accelerate when it gets to 4,000ish it will do it before changing into 2nd gear?!

I have tried reindexing the dizzy several times now, I can only assume I have it right or I am continually getting it wrong!

One thing I have wondered about is what timing advance these engines normally run? Out of interest I have pulled some of the data from the OBDII port, is this of any use to anyone?

When time permits I am planning on pulling off the rocker cover (I have a small oil leak at the gasket) and checking the camshaft with a dial indicator (I assume this is the way it is done), while I am doing this I will investigate the lifter preload.

Thanks!
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Re: Another high RPM power loss

Postby Cheromaniac » December 16th, 2013, 6:58 am

melgav wrote:One thing I have wondered about is what timing advance these engines normally run?!


10* BTDC at idle, 34* BTDC maximum advance.
1992 XJ 4.6L Stroker - AX15
202rwhp @ 4700rpm (248bhp)
258rwtq @ 3400rpm (311lbft)
1/4 mile: 14.63 @ 94.4, 3450lb curb
Stroker build-up, Stroker "recipes"
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Re: Another high RPM power loss

Postby melgav » December 16th, 2013, 10:44 am

At idle mine seems to run around 17 sometimes a little lower. The maximum I have seen so far is 41. Usually it runs between these figures.

Does this indicate something?

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Re: Another high RPM power loss

Postby Newcomerracing » December 16th, 2013, 7:52 pm

I have to agree with the dyno guys to some degree. Normally when a spring is cold it will not act up as bad, the more heat the worse they behave. The other thing would definitely be lifter preload, I had a sbc that had a flat issue and would not rev cleanly. I finally set preload from the original 1/2-3/4 turn to 1/8-1/4 turn and the engine ran great, I couldn't believe it. Possibly with heat the lifters are bleeding off to much. One other issue could be coil bind clearance, unless the heads spring pockets have been machined for more lift depending on cam lift and installed height this could pose a problem where the valve goes into float due to minimum coil bind clearance. Typically clearance needs to be atleast .050" or greater before coil bind occurs
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Re: Another high RPM power loss

Postby superstingray77 » March 4th, 2014, 8:54 am

Have you done a data logging session using an LM2 on it? When you put the engine under higher load the vac signal changes and thus affects timing and air fuel. When cold its in open loop mode thus ignores adaptive fuel trims. You can try cheating by moving the IAT sensor to a cold location like up in the grill. I would hit up Flyin Ryan OBD2 JTEC he is really really good. Just looking at AFR is only a part of the game. You need to see the timing curves the 99 up systems like to modify timing based on IAT etc. I had Ryan limit window in which the PCM can advance or retard the timing based on IAT after warmup because it would tend to over retard and kill power after it got hot or hotter outside. I moved the IAT sensor out of the engine bay while testing and re-logged it made a huge difference. So we mapped it out in the program and I installed the sensor back where it goes.
The 99up also has a torque management that does a timing retard on the shifts to try and help the stock 42RE trans survive. So that all needs to be addressed on a built up engine as well. Trying to get there with MAP adjusters/injector size/fuel pressure is low tech and compared to a full custom tune.... well trust me there is no comparison.
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Re: Another high RPM power loss

Postby 6TIME » March 4th, 2014, 4:43 pm

If you are not sure what springs you're running with that cam, you may want to remove one and put on a tester to confirm proper pressures. Was the cam degreed? To pinpoint your problem it's a process of elimination of potential variables.. Keep us posted!
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Re: Another high RPM power loss

Postby melgav » March 4th, 2014, 6:47 pm

Thanks for info. Been a bit busy lately.

At this stage I assume that the engine is fine, spring, valves etc. I bought the engine complete so I will give some faith to the builder.

I will try what stingray77 says as the problem does seem to be temperature related. I thought that it was open loop / closed loop related but I have since noticed that it takes some time before it has the "power loss" problem. The other reason I agree is it does not always do it, just most of the time.

I will report back after I experiment with the IAT mod.
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Re: Another high RPM power loss

Postby melgav » March 9th, 2014, 9:23 pm

I have tried relocating the IAT sensor to the front of the jeep. At this stage I have noticed a massive difference, thank you superstingray77 :cheers:

This seems to be my problem as the engine would only suffer the power loss when hot. The other morning when it was cold on the way to work the Jeep didn't do this power loss problem, but after driving around and allowing the heat to build up that is when it happens.

So my question now is should I just leave it as is?

If I leave the IAT sensor out the front is there any "other" problems i may run into?

Or should I fork out the big dollars (and they are big ones here in Australia) and get some sort of engine management / PCM tune / Piggy back?

Thanks Guys!
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