67 GMC

Having issues on my '87 Cherokee 4.0L / Auto. Seems to run well when cool. We've had some good weather and it was running great every day. Today is pretty hot and it was running fine and then suddenly it had no power and it would backfire through the intake if I stepped on it. Limped home and took a look at throttle body. Every clean etc. When I move the throttle manually it revs up and then starts bogging down. If I move it quickly, it will backfire. I'm thinking it's a hot-wire issue. I've re-done the grounds, re-did negative battery cable to block, cleaned up dipstick grounds, cleaned up block to firewall ground etc. I had routed all of the wiring away from heat spots etc. It has a new TPS sensor. I just re-did rear shoes, drums and wheel cylinders, front ball joints and u-joint, new e-brake, so I've got money in it now.

Is it worth doing a C101 bypass all together to elminate that as a source of problems? Would a bad O2 sensor cause this problem?

Thanks

cruiser54

Never hurts to eliminate the C101 completely, using solder and shrink tube only. No butt connectors.

67 GMC

Thanks for the reply Cruiser. I'm starting to think it might be one of the three sensors on the harness going to the bottom of the engine. The knock sensor, coolant temp sensor and O2 sensor are on that harness. Since it runs fine when cold, could it be the coolant temp sensor not reading correctly and telling the ECU to make it too lean or rich?

I measured the fuel pressure at the rail when I opened the throttle up while hot. The pressure is going very low (effectively to 0) but at idle, the pressure is normal with and without vacuum applied to the fuel pressure regulator. I was hardly opening it up though, maybe 1/16 of WOT. The fuel pump is noisy so it is on my list to replace. Could it be as simple as the fuel pump can't supply the required fuel when the throttle is opened up? My gut feeling is still electrical/sensor related issue but if I need to bite the bullet and put a new pump in that's fine.

I also have a vacuum gauge at my disposal but not sure what the specs are.

Any ideas, please chime in.

Thanks a bunch.

cruiser54

The fuel pump should never drop below the pressure specified for idle with vac hose removed. Address this next as a priority.

Knock sensor would definitely not be an issue.

67 GMC

I checked and re-checked TPS, cleaned IAC, re-routed/refreshed knock sensor, O2 sensor and Coolant Temp Sensor. Coolant Temp Sensor and MAT sensor are all reading the correct resistance for the temperatures (at engine cold and with engine hot). Drove for about 10 minutes, started acting up again. Made it home (barely). I put my ear at the gas tank and the fuel pump sounds like it's doing morse code. So I'm thinking the fuel pump is going from noisy to erratic. It actually sounds a little quieter lately so maybe it's as simple as that- just needs a fuel pump. Rock Auto has a Carter P74155 with strainer for $69. There is a Bosch as well without the strainer for $2 more. Taking the plunge and ordering. Let me know what you think. I know the Airtex is out!

Thanks.

cruiser54

Sounds like an excellent plan. Go for it.

DFlintstone

Hey...do you happen to remember the resistance you saw? Yea, I have the graph from the FSM and the chart....but my # where just close, not quite rite...They DO work...but IIRC for instance my CTS cycled up and down with the thermostat at 195*, in the low 300's

67 GMC

I had the table from the FSM for both the MAT sensor and the CTS. The table is the same (same ohms for same temp for both). I had around 2280 ohms at start up and around 200 when it was warm (200F). Both read in the same ball park.

DFlintstone

Rally! Well thanks so much....Maybe I should invest in a new CTS, might pay for it's self soon enough. I forget now, should have written it down...I think it didn't go much below 300. Thanks again!

cruiser54

I think I saw that note on your workbench in the photo you posted.........

67 GMC

Is this what you want?

DFlintstone

Thanks!

67 GMC

Updated-I ordered a Carter fuel pump from Rock Auto. Next time I will wait until I take something out before ordering. My Jeep had a Bosch in it. It took only about 15 minutes to take pump out and the Carter looks pretty similar. However, the rubber pieces and the way it is held in the assembly is slightly different. So, I put it in using the rubber pieces it came with and then I had to put a hose clamp around the whole thing as it felt to loose to me. In case someone else runs into this, the wiring is also a little different. The Carter has screw terminals and the Bosch has bayonet style. The smallest bayonet is the negative. Got it all together and put back in and it's working so far. I drove it for about 20 minutes and no strange acceleration issues. Tomorrow I'll take it for a longer trip to make sure the problem is solved. The new pump is a little quieter than the old pump but it runs steady.

I took a picture of the old pump.

DFlintstone

???? Glad you got it.....

67 GMC

I saw that as well but the '87 did not use Throttle body injection so the pump for an 87 should be a MPFI pump regardless. I had read on other formums that the different pumps were used by AMC so perhaps that's why they were completely interchangeable. I'm hoping my fix will hold.
There was a lot of deposits on the old pump so I'm thinking that it was just on it's way out. Plus the fact it was started to act up the longer I was driving. We'll see today-going to take it on the highway!