Stumble on Acceleration

[Porch951]

This is a 1993 Cherokee, 4.0L AW4, 170k miles.

I've been having a lot of stumbling (almost stalling) on acceleration. I read through a lot the threads here but nothing seemed to fit just right. So here goes!

-Stumbles more when cold
-Stumbles especially under higher load (going up hill, more throttle)
-Will stumble if i punch it at any rpm/speed
-Does not seem to be any specific throttle position
-No stumble without load (revving in the driveway)
-Could not find any vacuum leaks, intake gasket is newer
-Started literally overnight
-Put in a new o2 sensor 1 week ago
-Fairly recent tune-up (<10k plugs, cap/rotor looked fine, wires have maybe 50k)

I'm going to test my TPS for dead spots this afternoon, and i'll try to test my MAP sensor too. I was leaning towards the MAP but i also considered that my cat may be plugged (i have an aftermarket 2.5" exhaust, and the gasket on the flange before the cat recently blew out, which seems to support this theory). Anything i missed?

 

[Porch951]

Update: seems to run fine once warmed up.

 

[Porch951]

Annnd...now it idles really high when cold.

I went ahead and disconnected the battery to reset the computer (never did that when i put in the new o2 sensor). I'll see how that does. I also did some more searching and it sounds like a coolant temperature sensor could be my issue. I'll see how it goes if there are no responses here.

 

[magimerlin]

I would look at your IAC motor, MAP, TPS.... I would still test your new O2 sensor as well

Is there any check engine light on...if so have you had them read yet, if so what are they?

 

[Porch951]

No CEL before i unplugged the battery. After resetting everything, it actually runs worse than before... Probably a good indicator that one of the sensors has flaked out.

I know how to test the MAP and TPS, but does anybody know the resistance readings for the coolant temp sensor?

 

[yossarian19]

CTS resistance:
Temperature Resistance (k-ohms)
-40°F 291.49 to 381.71
-4°F 85.85 to 108.39
14°F 49.25 to 61.43
32°F 29.33 to 35.99
50°F 17.99 to 21.81
68°F 11.37 to 13.61
77°F 9.12 to 10.88
86°F 7.37 to 8.75
104°F 4.90 to 5.75
122°F 3.33 to 3.88
140°F 2.31 to 2.67
158°F 1.63 to 1.87
176°F 1.17 to 1.34
194°F .86 to .97
212°F .68 to .72
230°F .48 to .54
248°F .37 to .41
FWIW, years back I cleaned the connector on my temp sensor and cleared up a funky idle issue of my own. I might be ready to do it again, or chase some other electrical gremlin, but that is another topic.

 

[Porch951]

Thanks! Looks like i'll have some free time tonight, so i'll get it checked out.

 

[Porch951]

Alright, here's what i checked:

MAP: looks good. Has 4.0v out with the engine off, and with a vacuum pump drops steadily with no dead spots to ~0.5v at ~20" vacuum.

TPS: Seems like it might have a dead spot--BUT, the engine is currently dying at idle in drive (NOT at the potential dead spot on the TPS), so i don't think that's it.

Coolant switch-- i checked the one at the thermostat housing, is there a sensor somewhere? Seems fine.

IAC i'll have to check (could account for dying in gear, but i don't think it would matter on the part throttle stuttering, right??)

Vacuum leaks--i pulled all the connections off one at a time, they made no difference when plugged. Sprayed carb cleaner over every leak point on the manifold...no dice.

I'm going to rent a fuel pressure tester right now, might put in a new filter while i'm at it. Also considering that something ignition-related may be having trouble. Any other suggestions?

Thanks!

 

[Porch951]

Fuel pressure is fine (35 at idle, doesn't drop while driving).

I'm going to do a full tune-up so i at least have that squared away.

 

[Ecomike]

I forget what year the change happened, 91 or 96 (?), but the older fuel systems are 30-39 PSI, with a vacuum operated pressure regulator, 30 psi at idle with vacuum line attached, and 39 psi with vacuum line detached, IIRC.

The newer system had an FPR in the fuel pump, gas tank, and were constant 49 psi as I recall.

Either way it sounds like you have a FPR or fuel system problem.

 

[menos]

The issues you're having are similar, not exact, but similar to what I was fighting. In my case it turned out to be a bad fuel pump. Pressures were good when I tested it, but the flow rate seemed low using my admittedly low tech approach of dumping the return into a 5 gal bucket. It also seemed that the fuel was considerably aerated.

EDIT: Actually looking at all your issues and diagnostics again... It looks identical to my issues. Stumbling and high idle being the two noticeable ones.

 

[Ecomike]

Could be the hose on the pump coming loose in the gas tank.

Is the HO starting in 1991, 49 psi? or was it 96 ish?

 

[gradon]

Renix and "HO"/OBD1(91-95) have the fuel returns and are rated 30psi at idle and 39psi with the vacuum hose disco'd at the return. OBD2(96+) don't have the return and are rated 49psi +/-2psi.

 

[Ecomike]

Thanks, I never can recall the FPR and pressure increase change year.

 

[Ecomike]

So he said his was 35 psi, all the time, so it sounds like he has a bad FPR, and possibly a missing vacuum line at the FPR, for starters. Could also be a clogged fuel filter, or line plus a missing vacuum line to the FPR..or other combos.

 

[Porch951]

Sorry, i wasn't very clear. It was 35psi at idle with the vac. hose connected, which was in-spec according to the booklet that came with the pressure gauge i rented.
It varied with the engine revving a little, and remained steady in the mid-30's while taking it around the block. I don't think it's a problem, and the FPR was holding vacuum.

Since most of my sensors check out and i have no vacuum leaks, i'm going to treat it as a tune-up issue first. I'm going to pick up a cap, rotor, plugs, wires, and fuel filter today. I don't necessarily think that's going to solve my problem but it's probably due anyways and it's not that expensive to do.

 

[Ecomike]

Factory spec Fuel pressure at idle with vacuum line attached is 31 psi , and goosing the throttle up hard and then back to idle should run the fuel pressure up 39 psi, and then back down to 31, peak to peak with Vacuum attached. With vacuum line detached pressure spec is 39 psi.

 

[Porch951]

Factory spec Fuel pressure at idle with vacuum line attached is 31 psi , and goosing the throttle up hard and then back to idle should run the fuel pressure up 39 psi, and then back down to 31, peak to peak with Vacuum attached. With vacuum line detached pressure spec is 39 psi.

I didn't disconnect the vacuum line, but goosing the throttle did spike the FP up to nearly 40. I probably should have disconnected the vacuum...with the engine running as it is, i don't think the idle pressure reading is going to be what it should be. At any rate, the pump isn't dead and neither is the regulator, so i'm going to move on.

 

[Ecomike]

I didn't disconnect the vacuum line, but goosing the throttle did spike the FP up to nearly 40. I probably should have disconnected the vacuum...with the engine running as it is, i don't think the idle pressure reading is going to be what it should be. At any rate, the pump isn't dead and neither is the regulator, so i'm going to move on.

With the vacuum attached did it drop back to about 30-31 on deceleration? Or was 35 psi the low then too?

 

[Porch951]

I pulled the vac. line and got 39psi, but threw a fuel filter in there anyways.

Did a quick tune-up on it and it's running great now, but having trouble idling. I'm going to test the IAC tomorrow. The mid-RPM stumble is gone now and it's making good power (for a 20-year-old Jeep, i mean!).