zachtyler

1989 Renix 4.0. Over the past few weeks I have dealt with a weird issue. All will be fine, then randomly while driving it will start studdering/shuddering/feel like it's out of gas. At idle if the issue is present, it'll sound like it has a lope. If you put your hand under the exhaust you can feel the individual bursts of air, as opposed to a solid wave. TPS is in spec . I put a new o2 sensor in there. I thought maybe the MAP was bad. I replaced it and drove it for about 5 minutes before it came back. Replaced fuel filter, thought I fixed it because it felt good, then it came back as I pulled back into my street. The fuel coming out of my old filter was absolutely nasty, however. When the issue is present, the studdering/shuddering is most felt at maybe 1-15% throttle. That's where you can really feel it. But you can hear there's an issue and it feels like it's lacking power. Any ideas?

gat

You can "rent" a fuel pressure gauge for free from the parts stores. What is your pressure before cranking with the key in the on position, while cranking, while idling, and possibly while you are feeling this issue.

When is the last time that the distributor's rotor, cap, spark plugs, and their wires changed? Same with air filter. Also, did you confirm the direction of flow for the new fuel filter?

zachtyler

I will have to get a gauge. Entire distributor was replaced about 2 years ago. New spark plugs maybe 6 months ago. Not sure about the wires. Air filter seems fine. Fuel filter flow is correct. Any recommendation on plug wires? I will throw a set on either way. But I agree that checking fuel pressure is a must. School is wrapping up so I haven't had much time to work on it, only in town for this weekend.

Jimbo Slice

I had basically the same symptoms on my 89. Dollars to donuts says your fuel pump is on the way out. The gauge will tell the tale.

Good luck!

gat

Use whichever wires that you can get. Wires for spark plugs are one of the items that I suspect are massively rebranded across the spectrum.
I would be leaning towards fuel pump as well, but it could be that the throttle body cleaner needs to be cleaned with some throttle body cleaner/carburetor cleaner.

cruiser54

Every 30,000 miles you are due for plugs, wires, cap and rotor.

How did you adjust your TPS? Did you use my instructions?

It would be prudent for you to go to www.cruiser54.com and complete Tips 1,3,4, and 5 as a priority.

tjwalker

For your 89 4.0 to run properly, you need 31 psi. Yup, gotta test it with a fuel gauge.

Then disconnect the vacuum line leading to the fuel pressure regulator under the hood. The fuel pressure should jump to approximately 39 psi.

Check that vacuum line very closely for damage. And if there is any raw fuel inside of that vacuum line, you have a ruptured fuel pressure regulator diaphragm, necessitating replacement of the FPR.

Let us know what you fuel pressure testing reveals.

lawsoncl

It's an 89, so unless it's already been removed it'll have the white ceramic fuel ballast resistor on the front drivers fender. Try directly connecting the two wires without the resistor and see if that makes a difference. That'll give you the full 12-volts to the fuel pump. My other suspect would be the crank sensor, or cracked or dirty contacts on the bottom of the ignition module giving you intermittent or poor spark.

DFlintstone

Myself, here I'd start by gaping my plugs to .035, take it from there.....(the gap get's bigger each mile)

zachtyler

Fuel pressure read perfect. 31 psi with vac line connected, 39 with vac line disconnected. Gonna update thread. Check below.

zachtyler

Okay, fuel pressure checked out. 31 psi with the vac line connected, and 39 psi with the vac line disconnected. The problem has come back, not always but it comes back. I can say it feels like it's misfiring. If I push it while the problem is active (like climbing a hill) I will start to smell almost like a burned match/sulfurish type of smell. I looked this up and it looks like it could potentially be the cat. Would the backpressure from a clogged cat cause the engine to miss? I assume it can. Another thing I thought of is how my o2 sensor never really gets a full swing. Could the backpressure also be messing with the sensor not allowing it to fully swing? What do you guys think?

fb97xj1

A bad cat is a possibility.

If and when the substrate falls apart for whatever reason, a bottleneck is created. The exhaust backs up and the engine starts choking on its own fumes. It cant breathe, and perfomance suffers. The engine starts running hot, and misfires, which the cat cannot tolerate for long. Basically, everything is being cooked. If left alone the cat goes nuclear, turns cherry red, and the substrate literally melts. If it blocks off the flow completely the engine might start, but stall soon after. Or not start at all. If you suspect the cat, you can remove it and take a drive. If the engine runs better, youre on the right track. The question then would be why the cat failed in the first place.

Jimbo Slice

It absolutely could be a bad cat. Mine is gone, so it was easy to diagnose the fuel pump as the culprit... I forgot some guys still have theirs.

zachtyler

Update:

Just got the catalytic converter replaced. The old one was pretty much toast. While the vehicle feels does actually feel smoother overall, the original issue still remains. (I went in with the mindset that if it fixed the issue, it's just a bonus!) On the way to the shop, I've noticed that if I am cruising down the freeway for a while, it'll be fine. But as soon as I exit and idle at a stop sign or light, it will start to misfire. As I gradually increase throttle, it will have small bursts of total smoothness mixed with misfiring. Not sure where to start. I just got back in town, so I can work on it again. Still learning this thing and cars in general, so any advice on where to go from here would be appreciated.

Chick-N-Picker

Certainly won't hurt to bypass that resistor. I've had mine bypassed for 3 years just because I was having issues with fuel pumps. I bypassed it to rule it out and just left it that way.

Your problem sou ds exactly my 89 was doing when the fuel pumps (long story) would start to fail. Can you hear it whining?

You said your fuel and filter was dirty. If you do end up changing the pump. I highly recommend dropping the tank and cleaning it as good as possible and changing the filter again......been there done that.