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YJ stalls under load Howell TBI 4.2 manual transmission

666 Views 15 Replies 6 Participants Last post by  MrTrash

Recently purchased a 1988 Jeep YJ 4.2 manual with a Howell TBI conversion stalls under load. California Version.

History of the Jeep is unknown but has a Howell TBI that looks almost new, almost like it was never run. The vacuum routing was a bit wonky when I got the Jeep. The MAP sensor was wired but not connected to the TBI. The distributor was not connected to the TBI and there was a cap over ”filter to atmosphere“ port on the TBI. the other sensors all had power when running. the electrical connects all appear to be sound.

I have been through the Howell install manual and all the connections appear correct. I checked as best I could for vacuum leaks with propane. I connected a gauge to the constant vac source on the TBI and it held pretty steady at 15/in hg. there is power to the MAP sensor and good power to the three fuses in the fuse box under the dashboard. I replaced the fuel filter as a precaution and fuel pressure is strong (I could not get a good reading). The jets have a good spray pattern. I added about 5 gals of fuel just in case the fuel was funky. Distributor is pointing to the number 1 cylinder at TDC. The timing is about 8 degrees.

the engine fires right up when cold and idles at about 950 rpm accordimg to the dash tach.

i haven’t been able to pull any codes from the OBD1 port or check if there is any issue with the fuel return.

any ideas for me? the problem is so bad that the jeep stalls every time I try to leave the driveway.

thanks in advance.
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thanks for the replies. Based on the input here, I went back checked the timing and rechecked the vacuum at ‘A‘. I adjusted the timing and the engine seemed to run the best at 8* BTDC. The major finding was I found zero vacuum ’A’. To double check the gauge, I held the tube to my hand I could feel zero suction. When I revved the engine it would jump to 20 in/hg or so, but it was reading zero at idle.

I did the initial check in the dark after work and rushed it and didn’t notice my gauge was not zeroed.

I ran out of time and did not have a chance to pull the TBI to check the seal and if the RTV was mounded as Howell instructed. That bad of a leak makes me suspect that it is more than just the TBI. I will report back.

the good news is that I have darn near checked everything else on this jeep and otherwise all seems to be well. It has good compression and It appears that the TBI is plumbed and wired correctly.

thansk again for the valuable feedback.
@Shop Gumby clarification is needed. Not sure if he's still in the virtual area though.
Just saw this thread, (93YJ4FUN trying to interrupt my first coffee of the day?) and from a quick read-through a few things popped up. Regarding vacuum ports, the port on the rear of the throttle body between the fuel fittings is made for the MAP sensor, and the sensor only. Nothing else should be tapped into that line. Be sure to mount the sensor so it's above the port, otherwise the vapors, fumes etc., can accumulate oil and fuel in the line, eventually getting into the sensor. The front ports are a bit more complicated. There are several different port configurations on otherwise identical throttle bodies. They vary depending on their original application, vehicle etc., but almost all will work the same. The large port poking out the front center is for pcv only (C in casting). Directly under the pcv port and slightly to the right, looking at it from the front, is a ported vacuum fitting (A in casting). This port should have little to no suction at idle. It's routed through a slot/orifice just above the throttle plates when closed, which gives no signal. When the throttle is cracked open slightly, these slots are exposed to manifold vacuum. As the throttle is gradually opened all the way, they will eventually match whatever vacuum signal the manifold is making. The other two on either side (J and S) are where the variations come in. Some tb's have just a boss in the casting, with no fittings from the factory. I drill and install at least one before they go out to customers, since they both are designed for manifold vacuum. Sometimes GM installed just one. Sometimes they installed both. Sometimes they are both small fittings, other times they are larger. And there are also versions with both types, just to make things interesting. If the previous owner followed directions, the California emissions sticker included with the kit should be somewhere under the hood so you can get the vacuum line routing correct, and the brass E.O. smog tag should be screwed to the side of the tb just ahead of the throttle linkage bracket. I could keep blathering on, but those are the basics, and hopefully it will help get you going. I'll keep tabs on this thread just in case more blathering is requested :)
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If you have a scanner, you can also check running voltage to the fuel pump, tps and map sensors, and battery voltage. Might be a long shot, but if any of these are erratic it could be causing your issue. Also if voltage gets low enough, it'll affect how the ecm behaves. The official Howell test mule Jeep here has similar symptoms, but only occasionally and it's somewhere other than the fuel injection. (Can't be having that!) It gets irritating when plowing snow for sure, I'm thinking the distributor or maybe even the Duraspark module, since they are both original and have a LOT of miles. Keep at it, you'll sort it out (y)
- Dan
:coffee: for Gumby.
Thank you, fine sir. Now where did I put that Baileys.......
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