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Unread 01-08-2009, 09:20 PM   #1
WhrexKing
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1987 Cherokee total loss of power

This truck seems to be wanting to turn in to a huge moneypit for me. Fix one thing, something els goes utterly wrong.

It is a 1987 with Renix Injection on a 4.0L engine.

The starting issue it had, seems to have vanished, leaving behind this:

It's newest trick is, after it warms up, it loses all of it's power, and when I try and accelerate, it just spits and sputters when I stomp the gas.

It's really bad on the interstate when I'm going 65Mph, it has an extreamly hard time getting up past. if I stop for 5 to ten minutes, it's fine again for a little while, then the problem gradually comes back.

What would be causing this massive loss of power when it warms up? It idles fine, never stalls out, starts up just fine, even totally cold.

I've installed a few new parts to try and help the issue, but to no avail.

I've replaced:
Fuel Filter
Air Filter
Plugs
Wires
Roater & Cap
Vacuum Line to FRP

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Unread 01-08-2009, 11:49 PM   #2
balloo93
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Check the Muffler and cat and see if they are clogged.
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Unread 01-09-2009, 12:47 AM   #3
Razor.
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My '87 Wagoneer did that when the IAC was on its way out.

However I would also recommend you check your exhaust for proper flow.
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Unread 01-09-2009, 02:03 AM   #4
WhrexKing
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Cataylic converter is long gone, and my muffler system is screwed up. The bolts came lose at some point so the pipe is lose at the manifold where the two pipes clamp up. It's not overly loud so I just drive it like that. The bolts are frozen, so I couldn't tighten it up.

Also, the fuel pump is loud on and off, some days I never hear it, others all it does is whine and complain.

As I said, the issue disappears as soon as I stop and shut it off for 10 minutes. I can get it to do it even in park if I rev the engine up, so it's not like it's an issue that happens ONLY when I'm driving.
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Unread 01-09-2009, 02:53 AM   #5
Razor.
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Have you checked your fuel pressure? I believe the system should maintain 31psi
And it only happens after it has gotten warm correct?
Your fuel pump could be worn out. I would believe this to be the case if you are losing power during RPM's that require more fuel consumption then the pump can keep up with.
If that isn't the solution, it could also be a Crankshaft Position Sensor. Sometimes when they warm up they seem to encounter a thermal failure and cause a sputter or a stall.

Last edited by Razor.; 01-09-2009 at 03:04 AM..
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Unread 01-09-2009, 03:48 AM   #6
WhrexKing
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I'm thinking the pump too, because it's gotten to be extreamly loud, I've felt the lines as it pumps the gas though and I can feel a ton of vibration from it, that I have never felt on anything before.

Would also explain why it works fine when I stop for 10 minutes, the gas is ice cold right now, it's probably cooling the pump down in only a couple minutes so it works somewhat right again until it gets hot.

When I say it's loud, it's -LOUD-. I can hear it over the stereo now, and it is extreamly abnoxious.

I've done a lot of research on the cam sensor, and I do not have any of the conditions it causes. The truck has never stalled on me once, ever. it idles smooth as silk, and as long as I'm not jabbing the gas to get on a Chicago interstate, it runs smooth as silk.

I'm going to start with a new pump, I'm gonna spend the extra money and just order an entire new assembly, the old sending unit is trash anyway, so may as well kill two, or three, or more birds with one stone
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Unread 01-09-2009, 05:29 AM   #7
tjwalker
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I agree that the fuel pump is suspect. Testing with a fuel pressure gauge at the schrader valve on the fuel rail may confirm if you have a fuel delivery problem or not and what might be contributing to it. Don't forget though that you still have a fuel pressure regulator in the mix here.

If you are still symptomatic after verifying your entire fuel delivery system, it's time for more testing starting with the throttle position sensor. Your TPS can be tested and your vintage is adjustable; I'd want to be sure it is reading within spec.

Good Luck!
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Unread 01-09-2009, 09:30 AM   #8
WhrexKing
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I figure the same thing, because it didn't start to have weird starting conditions until the pump got loud. It's just graually getting worse and worse, so I think I'll start with a new pump & assembly and work my way in from there.

I had a blazer a couple years ago, act a lot like this when it had one of its 2 fuel injectors go bad. you'd floor it and the power was just not there. So it's acting like it's not getting enough gas to the injectors
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Unread 01-09-2009, 09:40 AM   #9
Razor.
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Sounds like a plan. If anything at least you will take care of other potential problems if it doesn't solve this one.
I have been fighting my old VW's carb forever so fuel delivery problems are always the first things I suspect.
"Door doesn't open? Replace fuel pump!"
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Unread 01-09-2009, 09:45 AM   #10
WhrexKing
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This is the first time out any car I have ever owned where the fuel pump was my primary suspect. Then again, the last car I drove that was this old, was a 1980 buick, and it didn't have a pump like this... This is EFI with a 22 year old pump
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Unread 01-09-2009, 11:33 PM   #11
WhrexKing
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Bump, because I just want to confirm before I drop $150 that I don't have to drop.

I tried the cheap route, and changed the fuel filter, and some weak dry rotted lines. No difference.

It seems to have gotten even worse today, it had a hard time keeping 55mph. I got off the interstate for fear of getting run over. I had to drive on slower roads for awhile, shortly before I got home, it couldn't even get up to 40Mph with out bucking.

I tried one thing as soon as I got home; I parked it, and reved the engine up, it spit and bucked even then. I shut it off, waited a second, turned the key BACK to ignition and let the fuel pump run again, stopped it, made it run once more and started it back up, ta-dah! Issue totally gone.

Is this a confirmed case of weak pump unable to keep the pressure up to the engines demand?
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Unread 01-10-2009, 01:30 AM   #12
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Yeah I would still think it's the pump. Your best test would be to get a fuel pressure gauge on it, that will give you a clear idea of how it is performing.
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Unread 01-10-2009, 04:29 AM   #13
tjwalker
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Quote:
Originally Posted by WhrexKing View Post
Because I just want to confirm before I drop $150 that I don't have to drop. Is this a confirmed case of weak pump unable to keep the pressure up to the engines demand?
The only way to "confirm" a problem is to test and get performance data. You can rent a fuel pressure gauge from many of the big box parts stores for a few bucks. Testing is easy and takes just a few minutes.

It may very well be your fuel pump, but if you read enough of these threads, you'll see that throwing parts at it on a hunch works far less of the time then does testing your way to a solution. Good Luck!
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Unread 01-11-2009, 07:43 PM   #14
WhrexKing
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Bump again since I'm starting to get really p***ed off. Next step is gallon of gas and matches.

Okay. I went out and got the new fuel pump today and went ahead and replaced it. when I removed the old one from the tank, I had made the discovery that the pump had once been replaced already, and had no strainer, and looked like a child assembled it. It was heled in place with PLASTIC TWIST TIES.

I went though all the proper steps and got it all back together the PROPER WAY. Truck runs fine now and the loud howling from the back end is gone, left with a light happy sounding hum.

I take the truck out for a good long ride today, taking it over 70Mph for a little over 20 miles; The problem showed back up, but it doesn't feel as serious as it did before.

When it starts doing this lagging on WOT, it has to be hot, and has to have been at high RPM's for a long period of time.

It feels like someone switching it on and off. Goes, bogs, goes, bogs, goes, bogs raidly.

What am I looking at here? I'm going to drive it home with the cam sensor unplugged to see if that has any effect.

What should I do next? TPS? Oxygen Sensor? What is more likely here with the described symptoms?

I'm really starting to dispise Renix.
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Unread 01-11-2009, 08:54 PM   #15
WhrexKing
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Update:

I drove home with the cam unplugged and it made no difference at all, infact it ran kinda lousy with it unplugged. I stopped and plugged it back in, and was fine all the way home.

I got home and started to read in to the Renix manual I have for the truck on testing fuel injectors, thinking it was one going bad. I read up on the TPS first and it clearly stated;

"A clear flood operation can be obtained by depressing the accelerator to the foor while cranking the engine. This will allow the engine to crank with out fuel delivery"

I used to have to do just that to get it to start up *AFTER IT WAS WARM*.

TPS is dead maybe?
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