x91j

hey guys im getting a wierd back fire through the intake of my jeep. it hasd the 4.0 also its a 97. when i start it it starts fine and i drive about a block and it sputters and backfires throiugh the intake for about i dont know 10 seconds then stops and everything is fine for the day. I checked fuel pressure today and it read 43psi at the rail at idle. reved the engine for a little bit and it didnt move from 43psi. Whats my problem? i replaced and tested the cps, tps ,map , replaced the iac and cts rotor plugs and wires and nothing has helped im thinking its fuel!

97 4.0 with auto trans and 164k

tjwalker

Backfiring through the intake usually indicates a "lean" fuel condition.

You have 43 psi of fuel pressure. Spec. pressure for a 97 is 49 psi, plus or minus 5 psi, which would put you at 44 for the low spec.

It is possible that your 43 is right on the hairy edge for giving you a marginal fuel delivery condition. And you can have a bit of inaccuracy in a gauge when measuring at well.

Snotzalot

I have seen lot's of posts over the years with this same issue, usually it's a bad sensor.

Intake air temp sensor checked? Since it only happens when you first start out it's not your fuel pressure, it's a sensor issue.

When it's misfiring, if you floor it does it take off with no misfire etc? If so it's a sensor issue.

Probably not, but when were your O2 sensors changed? They have a service life of about 40,000 miles.

Sicmade13

I Might not be allowed to be on this forum But I got a 1997 jeep tj 4.0l at with the same exact problem. I've been pulling my hair out for the last week. I've done plugs,wires,cap,rotor,cat,complete fuel pump ***, with new screens and fuel regulator, 2 new o2 sensors up and down stream, checked map,tps, checked for vacuum leaks. cleaned out fuel tank. The jeep will only back fire through intake when I first take off, Then it runs like a turd and gets crappy gas mileage the rest of the time. I thinking the computer might be going haywire. It has been in water pretty deep (before I bought it) I noticed the water line behind the dash. after I bought it. Now after a new trans and a whole fluid flush I been fighting this problem.

x91j

Ended up being a bad distributor and cracked coil

DFlintstone

I keep posting this same thing It was very hard to find and VERY easy to check/fix.

The injector wires come up over the front of the manifolds. The edge of that heat shield there can short the wires, a bit sometimes causing all manner of hassle, including backfiring.

I could start and run all day, no problem. Turn it off, grab a pack of smokes, take off, then it starts...things. The shield would heat up while it sat, expanding to make contact. SPREAD THE WORD! Tough to diagnose.

98xjclassic350

I am having the same issue right now! but mine is running rich! any ideas of how to fix it?

DFlintstone

I think maybe the Map sensor is suppose to do a smooth voltage sweep as the vacuum changes. Low vacuum to it from a bad line will make your Jeep run rich.

When mine was "Popping back", crapping out, and would go like hell if I floored it it was those wires grounding, it turns out maybe the ones to the O2 sensor.

Another guy had about the same symptoms and a new O2 fixed it.

Then again sitting, and heat soak is another issue. Having the heat shield on my 90 expanding into to wires seems to be really rare. Might be more common to be a problem with the CPS or fuel system.

You might want to start a new thread describing your problem and of course post what you have, then one of the "smart guys" here can probably help you isolate it. Don't let my high number fool you. Half of those where me asking the wrong question!

98xjclassic350

I actualy just did! I think I am going to try the map next! Thanks though

DFlintstone

One easy test is to pull the line off the map and see if it's sucking on your finger. My Map line was brittle and broke more than once. I guess my ECM feeds it 5 volts, then it feeds back 1 to 5 to the ECM as the vacuum drops when you put your foot into it.

Dirtdiggerjr

Mine was backfiring through the intake. Started with it only doing it when I first started driving then would not act up agin. But after a month of daily driving it got worse and developed a lack of power. Swapped my 703 back with stock injectors fix it for a few days. Then went right back to the same thing issue. So made me think fuel issue ordered a fuel pump off rock auto. Swapped it out in the drive way fired right up and now running like a screaming eagal!!!

DFlintstone

Much thanks! Lack of fuel seems to be the go to for that. Tough nut sometimes. One of the wires to the lower sensors grounding on a renix can cause that. (or maybe more than one).

Joe Lumpkins

I have a 97 jeep grand Cherokee. It wants to start but backfires thru the intake I have built the engine top to bottom and it backfires. Inset the timing as shown in the manuals I need help please

DFlintstone

The manuals are wrong for the plug wires on the dizzy. #1 is at 5:00 as viewed from the side, 153624 clockwise....

67 GMC

I'll go with fuel pump on this. Had this problem twice. Bad fuel pump each time. I suppose it could be clogged fuel filter too.