Renix Cherokee - Idles fine, bucks, shudders and stumbles on accel

So I knew this day would come sooner or later and just like the topic says my 1990 Renix Cherokee is now having this uncomfortable shudder, stumble and backfire issue whenever I try to put my foot into it.

Rather than asking for advice (Cruiser, I know what you're about to post, I already did that and I'm about to do it again RIGHT NOW! I would like to document my findings, as this is the third cherokee I've come across and I havn't seen a good writeup on this particular problem (yes I did google, and mostly it looks like it's probably a TPS problem). This is the third renix I've come across with this problem, the first one I was out in the desert on an off-road trip with some people I didn't know too well (owner had it towed out), second was a jeep I was thinking about buying for the wife (decided to pass), and now my jeep.

Signs and symptoms:

Over the last few weeks I've been noticing a greater incidence of "pinging" aka pre-detonation. I was considering that perhaps the knock sensor was bad.

Now, Tonight I was heading over to the shop, I put gas in it, started it up and tried to accelerate... no bueno, not happening.

Also, it's had an issue stalling randomly shortly after it's first start-up for the day.

Another thing that is probably unrelated, but could be, I've always gotten a persistent wine from my fuel pump depending on a number of things, I think this probably just means my ballast resistor is crap, but we shall see!

What I'm going to check:

Fuel pressure - I did a basic test, engine running in park, pulled the vacuum line to the regulator still stumbled on accel.
Fuel lines - I put a new fuel filter in about 3 months ago, but I will check and make sure I don't have a busted line that's letting air in/gas out.
IAT - I'm sure it probably needs cleaning at least.
IAC - I cleaned this out some months ago, worth a check
Throttle body - should be clean, will verify when I put a new TPS on it
TPS - I think this is where my problem lies, and I happen to have a spare so we shall see.
O2 Sensor - I have the one I took off a few years ago before my last smog check, I will check it with the MM and see if it's good enough to put back in. if the TPS doesn't solve the problem.

Photos Coming soon!

Fuel stuff all looks to be in good condition, can't find my spare filter, worry about that later.

IAT and IAC look brand new (from cleaning a few months ago)
Throttle body was a little dirty, cleaned it out with chemtool.

TPS - Well, like I thought, here seems to be the problem, I'm still checking it, but it looks like my B wire on the flat connector (4x4 automatic AW4) is measuring 6 ohms at the connector. I'm not really sure 1ohm is attainable, but we shall see, going to go clean the grounds according to:

http://www.naxja.org/forum/showthread.php?t=1072534

Also, I should note my battery connectors are looking a little goey, so I'm going to clean those too.

And for anyone who wants to check my math, I did have the battery cable disconnected earlier today because I was swapping my alternator.

Two halves to the TPS, if you have a AW4. The ground path on the two is different. On the engine (ECU) half, the ground is shared, you can't think of it as one ground for the TPS. Engine temp. sensor, MAT. MAP all share the same ground. And eventually all the injectors and others stuff.

Try doing some random ground checks on the other sensors, if you have splice issues this may help to localize which splice may be causing you grief.

I'd have a real close look at the harness up the front of the motor from the knock sensor, engine temp. sensor and O2 sensor into the injector harness. Numerous instances of this harness cooking on the exhaust manifold. People have a tendency to route it wrong during a intake/exhaust manifold gasket change or a water pump change. You really have to look close at this harness, the damage is really hard to see, you may have to disconnect the knock sensor, the temp sensor and the O2 sensor and pull the harness up the front to see the burn damage.

Jump a wire from the TPS ground to the battery ground and see what happens. I've never left that jumper in there, I fixed my ground/splice issues. But for a test, why not? If you run your TPS pigtail over the top of the rubber intake to the TB, things are much easier to test (mines been up there for 5-6 years now).

If you run out of things to look at, like faulty splices, check the injector harness where it makes the bend at the firewall. These wires have a tendency to fatigue and the copper wire (threads) separate under the insulation. The break can be hard to find, I usually find it by squeezing the wire between my fingers and feel the break. There is a TSB on this issue.

I fought with it for a while longer until it just got too cold for for me to stand around out there messing with it. I also figured out that my ohm meter was about 5 of the 6 ohms. So I spent the rest of the evening digging out my nice fluke meter that I will attack it with once the sun has been up for a few hours.

OOOOHKAAAAAAYYYY - So I went out and bought a HFT fuel pressure test kit, and at idle I'm getting about 15-20 PSI, so it looks like this is likely my problem (fuel delivery) glad I didn't get too far in messing with the TPS, not to say that's not still an issue, but FP would totally explain this. Looks like it's time to crawl underneath and check for a plugged fuel filter, bum FP regulator (I replaced this about 2-3 months ago) or just a bum fuel pump (testing electrics while I'm down there).

I'm hoping for a bum filter, I really don't feel like dropping the tank ATM, thankfully I didn't fill it up with gas last night.

Actually, quick question if anyone wants to chime in, specifically 8mud or Ecomike, the system is not holding any pressure unless it's running, I thought the FP was supposed to keep the pressure static? This may mean FPR.

Ok, So I've checked the electrical system going back to the pump, so far everything looks good. I'm getting 11V (dropped by ballast resistor, measures about 2.0ohm). This means either pump or filter are bad. I'm debating going over and picking up a new filter and pump before the autofarts store closes.

Volt tests are unreliable as far as testing the fuel pump goes. The reason is you can measure 11 volts through a single strand (thread) of wire, but are unlikely to get enough amperage to do you any good. It may in fact be the pump, it may also be any one of (I think) five connectors between the ballast resistor and the fuel pump, a wire or even a ground.

I always toss a battery on the ground and hook the pump up to that and see what I get.

You can also squeeze the return line partially closed with a pair of pliers, with tape wrapped around the teeth and watch to see if your pressure builds. If you close it all the way shut, it will build to around 90+ PSI and may damage something.

There is a check valve in the pump that is supposed to hold around 20 PSI when the engine is shut down. These are notoriously unreliable and most will leak down to near zero in a few hours. The fuel regulator is what regulates working pressure.

Pumps are expensive and a pain to change, I always test them out well before buying a new one. And the new ones are notoriously unreliable which is another reason to keep what you got if possible.

The first fuel issue I had with my Renix was a bad connector near the C pillar. The next was damaged wiring from a tree branch back by the pump. The third was jello (varnish) fuel in the fuel line, I blew out the fuel line with compressed air and had my injectors cleaned. I have no idea how the varnish/jello clumps of crap got into my fuel line. The fourth was about five pounds of ice in my fuel tank. The last time it acted up I could hear the pump wasn't sounding right. I whacked the side of the fuel tank a few times with a rubber mallet and I evidently dislodged something, because the pump has been fine since ( about two years ago).

Pull the filter and bang the inlet side into a plastic dish a few times and see what comes out, blow in the outlet side. Half a teaspoon of rust flakes come out or clumps of goop and you've likely found your problem.

It's always possible the rubber lines near the pump are rotted through because of Ethanol.

If the pressure builds when you squeeze the return line partially closed and the fuel pressure doesn't change when you pull the vacuum line off the regulator, I'd be looking at the regulator instead of the pump.

I just got back from vatozone with a new fuel filter, going to put that on in a second.

I just hooked the FP-gauge to the rail and was going to jumper the pump "on". The suggestion you had about clamping the return hose should tell me whether it's the regulator or not. I think it's been about 2 years since I last replaced the filter, and at the same time I also redid all the rubber so it's got new lines front to back in the last 2 years. Also, I usually run non-ethanol fuel in this as it boosts my mileage up to 22mpg and gives me fewer headaches over-all.

What currently makes me think it's the regulator is with the FP-gauge hooked up, I turn the key to "run" and it pops up to 30PSI and then immediately drops down to about 5PSI. I tried to go get warranty service over at napa (where I bought the reg I put in a month or two ago), however they seem to keep bankers hours and were closed at 6:15pm. One of several major complaints I have about napa...

When I redid the lines, I dropped the tank and fixed the rollover valves on top and the vent lines, at the time my FP looked really ugly, like a gradeschool science project, covered in zipties with wires hanging out all over the place, kinda felt a bit nervous plugging it back in, so before I put fuel back in, I purged the whole thing with carbon dioxide.

In other news, my cousin apparently works at Onix, I guess they make fuel pumps, see if he can get me a deal.

Update:

New fuel filter installed.

Jumpered fuel pump, clamped return hose, pressure rises to about 30PSI and then falls off. I still hear it running however I wonder if it stripped the impeller off the motor or something. Either way, looks like I'm pulling the tank tomorrow whether I like it or not. In the mean time, since the FP works (sort of) I'm going to use it to pump all the gas out right now.

Anyone have a favorite fuel pump?

Pull the pump and check the rubber hose on it, i had the same issue this fall and discovered the clamp was loose.
As for pump get a bosch and don't look back.

heshhash said:

Pull the pump and check the rubber hose on it, i had the same issue this fall and discovered the clamp was loose.
As for pump get a bosch and don't look back.

Click to expand...

You win the prize on this one, the hose had rotted out and slipped off. I've got pictures I will post a bit later. I'm still putting the filler neck back in.

Don't you love days of frustration over a 60cent fix.

BulletMaker said:

You win the prize on this one, the hose had rotted out and slipped off. I've got pictures I will post a bit later. I'm still putting the filler neck back in.

Click to expand...

The new fuels, coupled with old hoses, wreak havoc.

So, kept the fuel pump and just replaced the hose?

How's the whine sound now?

I replaced the FP since I had it open, and keeping the one I replaced as a known good spare in my emergency kit.

Whine is now gone, I can hear it a bit when I'm right next to the tank, but that's it. I need to pull photos off my camera for posting.

Okay, so you replaced the pump AND the hose.

Yes, I should probably also mention that the pump I took out, before I put the whole thing back together I did a bench test on it, and it ran the pressure up past 80PSI before I shut it off (pumping gas from a cup right into the test gauge)

So the hose was most likely the culprit.

I was wondering if a bad hose would cause whining also.

I imagine so, because it would be running harder and more than it would if the system was operating normally. Also here are the photos of the final battle:



That's what it looked like when I pulled it out.

This is what happened when I pushed on it slightly with my finger:

BulletMaker said:

I imagine so, because it would be running harder and more than it would if the system was operating normally. Also here are the photos of the final battle:

That's what it looked like when I pulled it out.

This is what happened when I pushed on it slightly with my finger:

Click to expand...

No pics came through.