Stumble Sputter Stall Bog Surge and Rev

[Gubachoo]

Hey guys, having an issue and spent a while combing through previous posts looking for help, but no one seems to have the same issue, so I figured I would put it out here and get some opinions. 

 

I have an 88 with a 4.0, AX15. She starts just fine almost all the time, and idles well initially, but the as soon as you start off the clutch she stumbles hard and its a task to keep her from stalling. She's very finicky with how she responds to the throttle at different times. She surges just barely noticeably while at speed, but gas mileage hasn't suffered even a little bit. Smells a bit rich at idle. Now the strangest part is she will often drop very low when the clutch is pushed in while slowing to a stop at a light, if she does stall, I usually just pop start her and here is the odd thing that I have played with a lot. Once she starts back up after the pop, she jumps up to 2500RPM or so, and doesn't want to come back down. What I can do to get rid of this is (while still in gear) cycle the ignition quickly, and the high idle disappears, which eliminates anything mechanical it seems. I experimented the other day, and if I pop start like I described before, and it rev's high, I can head down the road at a consistent speed (where those revs want me which is about 45MPH in 4th gear) and it will not drop down on its own. I have braked to lower the RPM's below 1000, trying playing with the throttle a bunch of ways, nothing changes it. But if I leave her in gear, and quickly cycle the ignition, BAM, all gone. Now, that's not to say she is running well, I can still feel the surge and the throttle response is lacking, but the idle is at least between 500 and 1000.

 

Now, as for what I've done so far. The MAP sensor is good (traded with a known good from a friends cherokee and no change) and the map sensor tube is good as well. Same with the IAC. Have cleaned the throttle body and plates. The EGR seems to respond the throttle input normally, but I also know that will change with load on the engine, so I can't be sure, but as this is not electrically controlled I can't imagine why the ignition cycle would affect it. That being said, I also know that this could be two separate problems interacting with one another so I'm not willing to discount EGR quite yet. I have ordered an O2 sensor as I have owned the truck 5 years and know it needs to be done as a maintenance item at this point (lord knows when it was done last) but am not quite sure where to go from here. I think I will start checking sensor input voltage at the MAP and others to eliminate some of those potentials. 

 

No weather or humidty or temp seems to change this. Sometimes it idles great at lights (except for the slight surging/stumbling that often passengers don't notice because it is so slight) and sometimes I have to keep tapping the gas to keep it from stalling. Very hit or miss.But, to be clear, even when it is idling better, it is most definitely not idling correctly, just better than when it is struggling. It starts just fine and usually idles well for a short time, which makes me think the O2 has something to do with it, since I believe she goes closed loop after 30-60 seconds. but this still doesn't explain the revving. 

 

I have already eliminated the C101, upgraded all my cables and grounds (cleaned all contact points, beefed up cables) have checked for vaccuum leaks, have a new(ish) ignition system, and have given most electrical connections a good once over. I am not sure where to go next, and as I have little money I don't have the ability to throw parts at it (and as an ex mechanic, hate the idea of "well, I'll just try this, can't hurt")

 

I can't imagine how this contributes, but in the interest of full disclosure in case I am missing something, she has a porous freeze plug under the manifolds so I need to add coolant every few days, and she will run hot sometimes if I don't add it when I should, in which case I shut it down, let it cool, and top it off before I carry on. I hope to fix this in the fall, but it looks like either the manifolds come off, or I pull the engine and just do all freeze plugs and most gaskets to be done with it. 

 

Any suggestions or further areas of inspection are appreciated and I'll fill in on whatever I have done that I forgot to mention. Thanks in advance for any help.

[omega_rugal]

coolant / air temp sensors are ok?

[Gubachoo]

coolant / air temp sensors are ok?

 

 

Don't know, didn't consider those as a possible source to the problem. Any reason you would suspect those? I'd be willing to check, just don't want to go testing things which may not be related to the problem. Thanks for the input.

[Jeep Driver]

Yes, temp sensors are relevant.

[Gubachoo]

Yes, temp sensors are relevant.

 

OK, great. Does anyone have the Ohm specs for them hot and cold?

[Minuit]

Check the vacuum line to the MAP sensor on the firewall again. I had this exact problem on my 89 and it turned out that the vacuum feed line to the MAP had a crack in it. Check it very closely, the crack in mine was VERY small and I had to look very carefully to see it. The sensor is very sensitive. Ohm specs for the sensors coming, gotta look at my FSMs.

[Minuit]

Coolant temp sensor resistance values (intake air sensor will be the same)

 

Temp *F  Ohms

212  185

160  450

100  1600

70    3400

40    7500

20    13500

0      25000

[Jeep Driver]

Renix temp sensors are still available from the dealer, I just bought one, $25. 

 

 

Not worth the effort of testing..........I'm suspicious of all the science projects round here anyway. :P

[Gubachoo]

Check the vacuum line to the MAP sensor on the firewall again. I had this exact problem on my 89 and it turned out that the vacuum feed line to the MAP had a crack in it. Check it very closely, the crack in mine was VERY small and I had to look very carefully to see it. The sensor is very sensitive. Ohm specs for the sensors coming, gotta look at my FSMs.

 

Alright, I will go double check that line now. I will be near my multimeter tomorrow and will check those values. Thank you for looking those up. Would be lovely if it was something as simple as one of these. 

[omega_rugal]

all about sensors

 

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

[Gubachoo]

Well, this took much longer to get to than I anticipated. Anyways, I just found that the TPS output voltage starts at .85v at idle, then dives to close to zero off idle, until about 15% throttle where it behaves normally, so that explains the hard stumble off idle. I'm about to head to the parts store to grab one, but wanted to put a question out there. I checked cruisers tips about the TPS and it says it is a 3 wire for 5 speed (like mine) and a 6 wire for autos. Thing is, mine is a 6 wire AND the second plus has a corresponding plug which heads into the main harness and back towards C101. The truck is definitely a stock 5 speed and the harness shows 0 evidence of butchery. I am hoping the parts store has the 6 wire for the same or close to the three wire as I am wary to leave it unplugged if it was there stock. It very well may do nothing, but for a 30 minute trip to the store, I would hate to put the 3 wire in, have something not jive, and make the trip again. Maybe I'll just get both and return the other the next time I head to town, but it is a little bit of a headscratcher... Maybe its just the general harness and the secondary part of the TPS wiring terminates in a connector somewhere? Anyway, I'm off the the store. Anyone run into this or have any insight?

[SVPete]

I am the original owner of my 1988 5-speed manual Pioneer. Following advice gleaned from this forum, I have an automatic transmission TPS installed (two plugs, maybe six wires, never bother to count) since they presumably work the same and cost a lot less than the manual transmission TPS. My engine harness also has the corresponding connector, but I don't have it connected.

 

The truck works fine.

[cruiser54]

3 wire flat connector is for the engine side. 4 wire square is for the trans. 

 

Auto TPS works fine on a manual trans equipped vehicle. 

 

Have you done Tip 5 yet?

 

Do it before installing the new tPS and use my instructions to adjust your new tPS.

[Gubachoo]

Sorry to take so long to repost, I'm traveling and forget. Anyway I replaced the tps, clocked it as recommended by your (cruiser) tip, and that problem is now gone. To answer your question, yes, I cleaned and checked the grounds when I upgraded the cables and removed the c101.  I am happy it works, but just wonder why a stock 5 speed has the two plug with the six wire? No big deal, just curious. 

[Kerry]

I cannot say this for sure but my guess is it's easier and cheaper to use one wiring harness on both autos and manuals than to have two different harnesses. AMC was very good at using parts that would work on a variety of different applications.

[cruiser54]

4 minutes ago, Kerry said:

I cannot say this for sure but my guess is it's easier and cheaper to use one wiring harness on both autos and manuals than to have two different harnesses. AMC was very good at using parts that would work on a variety of different applications.

You are correct.