Problems with new engine, need HELP!

[Firemanray]

Back in June my son blew the engine in the Xj, it was a total loss. Matt donated an engine to replace it so I took it to Thunderbolt to have it freshened up since my work load was overwhelming at the time. I finally got the thing back together last week and tried to start it last Saturday only to find the fuel pump was out. Today I replaced the pump and the engine fired right up and idled fairly good. Now when I throttle up the engine it revs slightly for an instant then starts spitting back through the intake like slow timing or lean condition. I checked the setting on the distributor and it is correct, firing order is correct. I'm not sure what to do next. Open for ALL thoughts and suggestions. O gotta get this thing going so we can all have our vehicles back!!!

[Mudderoy]

Back in June my son blew the engine in the Xj, it was a total loss. Matt donated an engine to replace it so I took it to Thunderbolt to have it freshened up since my work load was overwhelming at the time. I finally got the thing back together last week and tried to start it last Saturday only to find the fuel pump was out. Today I replaced the pump and the engine fired right up and idled fairly good. Now when I throttle up the engine it revs slightly for an instant then starts spitting back through the intake like slow timing or lean condition. I checked the setting on the distributor and it is correct, firing order is correct. I'm not sure what to do next. Open for ALL thoughts and suggestions. O gotta get this thing going so we can all have our vehicles back!!!

O2 sensor? I guess you're not getting any codes? Did you go through the CAM lifter break in procedure? Not that has anything to do with you backfire...

[XJ Wheeler]

Distributor turned 180° can cause a backfire through the intake. Also the rockers not set right.

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[Firemanray]

O2 sensor? I guess you're not getting any codes? Did you go through the CAM lifter break in procedure? Not that has anything to do with you backfire... CEL is on I'm sure there are codes. I don't think O2 sensor would cause this extreme condition. Idle only, no rev, spits back when you try to rev.


Distributor turned 180° can cause a backfire through the intake. Also the rockers not set right.

Sent via messenger pigeon - i talk, he types.
Distributor is correct. I tried to start it 108 out before and no go. Rockers are "fixed" position, no adjustment.


Thanks for the suggestions I'm sure someone will come up with something that will fix it. I may not be describing the symptoms accurately, however when the right "fix" comes along a light will go off in my mind.

[XJ Wheeler]

CEL is on I'm sure there are codes. I don't think O2 sensor would cause this extreme condition. Idle only, no rev, spits back when you try to rev.


Distributor is correct. I tried to start it 108 out before and no go. Rockers are "fixed" position, no adjustment.


Thanks for the suggestions I'm sure someone will come up with something that will fix it. I may not be describing the symptoms accurately, however when the right "fix" comes along a light will go off in my mind.

I mispoke on the rockers, my meaning was are they seated right and tight?

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[Firemanray]

Additional info: Checked for vacume leaks today, found none. Engines acts VERY lean, spits back like an old carbed engine with a bad accelerator pump or like the timing is very slow. I wonder if the TPS is working, it acts like it doesn't "know" when the throttle is advance. New thoughts??

[bigjim350]

Did you check the timing with a timing light? Cause the distributor can easily go in one tooth off.

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[Mudderoy]

Additional info: Checked for vacume leaks today, found none. Engines acts VERY lean, spits back like an old carbed engine with a bad accelerator pump or like the timing is very slow. I wonder if the TPS is working, it acts like it doesn't "know" when the throttle is advance. New thoughts??

This same thing happened when Matt and I started my 4.0 for the first time. I can't remember what it was. I think it may have been what Jim said, distributor was off a hair. I remember Matt stabbed that thing several times.

[Mudderoy]

Yep, just double checked with Matt, that was the problem with mine. It was a tooth off. About a week ago I damaged my upstream O2 sensor and it was acting up. I could get it past idle though.

[Cheromaniac]

I checked the setting on the distributor and it is correct, firing order is correct. I'm not sure what to do next. Open for ALL thoughts and suggestions.

It does sound like you have a lean condition so here's a few things to check:

1. Distributor indexing. It could be off one tooth as others have suggested so recheck it. This article might help:

http://www.angelfire.com/my/fan/dist_index.html

2. TPS. Backprobe the middle wire and with the ignition on/engine off, check the voltage. It should ramp up steadily as you gradually open the throttle. Any sudden spikes or troughs indicate either a bad TPS or a faulty connector.

3. Check the resistance across the coolant temp. sensor and manifold air temp. sensor (see attached chart).

4. Fuel pump. Is it brand new? You might wanna test it anyway.

5. Go to Autozone and have your ECU scanned for codes.

[msmoorenburg]

Ray give this a once over and look for the hole in the dist.

http://www.xjtalk.com/showthread.php?t=10203

[Firemanray]

Ok here's the latest. I checked the distributor and the timing with a light, I'm 100% in that department.

While the engine was idling I unplugged each injector one at a time just for grins, not much change in idle quality or speed. I'm convinced at this point it is fuel related so I unscrew the schrader valve cover and poke the valve while running, a little fuel dribbles out and the engine dies. Now I have to get a valve pressure gauge and see what the pressure is. I'm afraid the BRAND NEW pump is not doing it's job. I also noticed the fuel gauge is inop since I changed the fuel unit. Looks like I may have a bad fuel pump assembly, I'm glad I haven't re-installed the fuel tank yet!

[Firemanray]

Another update. I managed to find about 15 minutes to go to the shop and put a fuel pressure gauge on the Xj today. I did not start building pressure until after the engine had cranked for a bit, then it climbed slowly to 50 psi and the engine started and ran fine at idle. The pressure then slowly began dropping until it leveled out at about 15 psi. The engine began to idle rough then died. I haven't checked the line from the tank to the fuel rail for restrictions yet but I'm going to change the fuel pump AGAIN anyway since the fuel gauge didn't work with the new pump assembly. If the newest pump acts the same I'll check the pressure at the tank and then down the line to see if there are any restrictions (i.e. kinked lines etc.). I'll report back the findings.

[Firemanray]

Problem resolved. The new fuel pump assembly resolved the fuel gauge issue but as I was finishing the pump install I followed the fuel line along the "frame" rail towards the engine. Just as the body makes the transition from flat drivers floorboard upwards toward the firewall there is a goofy piece of hard "plastic" section in the fuel line. During the engine swap the fuel line was disturbed causing the plastic fuel line to kink. The kink would allow fuel pressure to build but because of the restriction the volume of fuel was not enough to maintain pressure at the fuel manifold. The beast is back together and ran awesome on it's shake down run. The thunderbolt engine is smooth, quiet, and makes plenty of power. The boy has his ride back and I can go play with my 'vette now!!!