I finally got the 1990 Cherokee I6 to run after replacing the CPS, fuel filter, pump, distributor, cam sensor, etc...But now it backfires. It's not consistent either. It idles fine around 900rpms, but when I put some gas to it quickly it backfires sometimes, if i gradually do it it wont backfire as much. It back fires in the manifold it sounds like, although while testing I was once able to get it to backfire out of the exhaust.

While I was trying to get her to run I replaced the injectors with the Neon 703's. Maybe they're too much for my engine?

I fixed any vacuum leaks that I can tell. I don't think it's timing otherwise I would believe that the idle would be crap.

I pushed the TPS lever down with a screwdriver while it was idling and noticed it ran a little rough.

Any ideas?

 

My money would be on distributor being set wrong.

Did you line up the mark on harmonic balancer on intake stroke?

 

I thought I did, I moved the balancer to 0 on the timing marks, set the distributor to 5 o clock, when it sat it was at 4 like it says in the FSM. I had heard that if the distributor gear is off even a tooth the engine wouldn't start.

 

My XJ still backfires so I know what your going through. I have replaced every part possible but the MAP sensor. That's my next move at least. I will let you know what happens.

Mike

 

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

 

So, I followed this page even though it's for an H.O. and there are some differences. Now the thing won't start at all.

I brought the balancer to 0 and instead of taking the valve cover off I just took spark plug 1 out and looked when I saw the piston. (the page says that if the balancer is on 0 and the valves are open to turn it another 360, which doesn't make sense to me. Every time the tick mark hit 0 I could see the piston coming up

Anyway, the H.O. distributor has its cam sensor wires almost right under the 1st plug, whereas on the non-
H.O. the wires come out under the 5th plug. Anyway, according to everything I've read, if you're looking at the engine from the pass side the rotor should be pointing at 5'oclock with the trailing edge of the rotor just touching the 1st plug on the distrib. That's where I am and I have no start at all now. I've now reindexed this ****ing thing a half dozen times and the 7th time just turned the shade of my blue face a little darker.

 

I re-re-indexed it again and it starts again. I've indexed every conceivable way and this way, according to the FSM is the only way and it starts again, but it's still back firing.

Idles fine. Still just stutters and backfires between the 1200-2100 rpm range. If I floor it, it's fine. When sitting at park and I very slowly bring the rpms up it will usually not back fire, but not when it's in drive, it back fires everytime.

Think I've ruled out a misindexed distrib. Square 1 anyone?

 

On engines with a distributor there must be a device to advance the spark. Since you said it idles fine ,that maybe your issue.

 

The ECM does the advancing and retarding, theres no manual way to do it on a I6 4L engine. At least from everything I read.

BTW, put new plugs in, didn't help. Went with champions RC12Y's or whatever, they're the copper plus ones.

 

Here is an interesting article on engines that backfire.

http://www.2carpros.com/how_does_it_work/backfire.htm


Mike

 

Did you mark the fuel injectors harness for position? Maybe you have two switched.

Firing order is correct?

 

I was worried about the injector order and found the wiring diagram. They're fortunately/unfortunately right. (wish it was that easy) Also, the distributor is setup as 1-5-3-6-2-4 as it is stamped on the throttle body. Again, wish it was that easy.

I'm really beginning to think these injectors are too much for my old engine. I think I'll step away from it today for the Cherokees and my healths. So sick of this crap.

I read that article from 2carpros too. I'll probably read it again tonight and take some notes so I can take steps tomorrow.

Just for grins, I think I'll disconnect the O2 sensor to see if anything changes.

 

put your thumb over the spark plug hole and turn the crank with a beaker bar. when the air in the cylinder is pushing against your thumb, trying to blow it off the hole, you are on the compression stroke. this is where you need to be when you set your distributor and crank together.

also, since we have a distributor, there should be an a timing advance control in the distributor. it is either mechanical in the form of some springs with wieghts (fly wieghts) or vacuum controlled. i don't remember seeing a vacuum line hooked to the distributor though

 

I forgot that step with the thumb. FrankZ told me the procedure a long time ago. I guess I'll reindex it again adding that to the routine.

I sincerely believe there is no manual timing advance on our distributor. I know there is no vacuum connection and the distributor only has the camsensor/pickup coil in it.

 

i didn't catch the part earlier about the ecu controlling advance.


try the thumb way and then tell us what happens