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Unread 01-09-2011, 02:40 PM   #1
underpowered
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1990 renix 4.0 runs like crap

no better way to describe it. all started new years day. was out wheeling, after a long steep climb turning high RPM's the whole way up it began to run terribly. the fan contacted the radiator and put a hole in it, so my initial thought was wet electrical system, no biggy.


Today i repaired teh radiator, rolled a core or two on it to stop the leak, good as new. fired it up and still ran horribly. checked the oil, not milky so a head gasket is not looking like the culprit. pulled plugs to see what they looked like.

first one i pulled is not a good sign, the electrode has had physical contact of some sort and has 0 gap. pulled the other 5, all look like it is running a bit rich, but not bad at all. for all i know, the first plug may have been that way from when the PO gave it a tune up. new cap, rotor, and plugs later, it seemed to run great for 20 seconds or so, then back to its old ways of missing, coughing, nearly dying at idle, along with some valve noise as well.

gonna replace wires tomorrow, but after that i am stumped. fuel pressure looks good as well, not sure what it is supposed to be, but i figure 50lbs at idle should get the job done.

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Unread 01-09-2011, 02:43 PM   #2
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check the vacuum line to the MAP sensor. that one little line will make the jeep run rich if it is leaking or not connected. if the line is fine, then maybe that sensor is bad. also, has the o2 sensor been replaced lately?
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Unread 01-10-2011, 05:54 PM   #3
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o2 has never been replaced. but it is not excessivly rich. richer than ideal for sure, but not near bad enough to make it run like it is. only light hints of black smoke when revved. rich enough to make it run like it is, this thing would look like a diesel.
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Unread 01-10-2011, 05:57 PM   #4
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1. Finish the tuneup by replacing the wires, cap, rotor.

2. 50 pounds of fuel pressure? Measured with a fuel pressure gauge? Not good! For a renix, you should have "31" psi at idle. Then disconnect the vacuum line to the fuel pressure regulator and the pressure should jump to approx. 39 psi. Be sure that the vacuum line leading to the regulator is pulling good vacuum and is not WET with fuel. If it is, you have a ruptured diaphragm in the regulator and you must replace it.

3. Here is a link to an article that can help you with testing many things on your RENIX.

http://www.lunghd.com/Tech_Articles/...iagnostics.htm

Good luck and check back in and let us know how it goes...........
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Unread 01-10-2011, 06:03 PM   #5
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yupp 50 psi. and it has a new cap/rotor just not wires. worth a shot to replace them i guess, but all tested fine. insulation is good on all, 15kv is when they blew at on the electrical insulation tester at work, which is more than a new one blew for my truck

then on to the fuel system. might check pressure with another gauge to confirm though.
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Unread 01-10-2011, 06:27 PM   #6
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Definitely confirm. 50 psi is out of spec. More is NOT better with fuel pressure.

You could easily have a fuel pressure regulator problem here.
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Unread 01-10-2011, 06:36 PM   #7
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definitly worth a shot.
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Unread 01-10-2011, 06:50 PM   #8
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check the vacuum line to the FPR
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Unread 01-10-2011, 07:53 PM   #9
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gonna check FPR tomorrow for sure.
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Unread 01-11-2011, 03:11 PM   #10
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checked pressure again today. 41 whiel running, pull the vacuum hose and it jumps to 49. TJwalker, i think you maybe 10PSI off on your specs, cause i looked up a new FPR and it says it regulates to 41, in that case i am spot on.

but meanwhile it seemed to run better today, took it out for half an hour or so in the snow, and other than an excessively high idle it ran great. and i believe the idle to be a bad TPS.

however, my Rad fix did not work, still has a slow leak so time for a new one.
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Unread 01-11-2011, 04:53 PM   #11
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Pretty sure the spec for the RENIX (87-90) is 31 psi at idle. 39 psi with vacuum line disconnected. If it is incorrect, I've been giving the wrong advice for years!

The spec needs to come from the manual, as what the FPR can regulate to is not the actual factory spec.

Someone else please chime in if I'm wrong.............
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Unread 01-11-2011, 06:08 PM   #12
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hmm, found conflicting answers online.

on site claims exactly what you are 31 connected, 39 disconnected.

while another site claims 45 and 54. but as i said, the regulator i found says it regulates to 41 as a set output pressure.
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Unread 01-12-2011, 04:04 AM   #13
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The spec for 45 psi and 54 psi is for the 97 and newer.

Your spec is 31 and 39 with FPR disconnected................

There is a big difference in the design of the older and newer fuel pressure systems; therefore the difference in high / low specs.

Are you sure that your FPR SETS the pressure to 41 or does it "cap" the high pressure at 41. Big difference. May want to double check the part number for correct application.
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Unread 01-12-2011, 02:19 PM   #14
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might try a new FPR just in case, but hate to throw money at it without knowing it is gonna fix it.
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