SirCyber

Hello all, first time posting in this section but I my dad has a 98 grand that needs your help. I have a hunch but I will reserve for end of post.

About 3 months ago, dad had a friend replace his exhaust manifold due to some really gnarly cracks in old one. Dad says it's a Borla off of Amazon. Ever since, it has run like doggy doo. I got to hear it, and this is my observation.

On first startup, there was a big backfire from front of engine. Dad said it runs better with fuel injector 1 or two unplugged. I tried that but we couldn't get it running, with either injector unplugged or all plugged in. Some friends said it has an exhaust leak. With the backfiring, and running absolutely terribly, I think it's the throttle position sensor. Also the header didn't look like it matched up correctly. It's a single piece header, but I thought 98 was a two piece? I'm not savvy on newer 4.0's, so I'm most likely mistaken on the two piece thing. So, thoughts? Throttle position sensor? Could that tell the injectors to inject at the wrong time, squirting gas on the exhaust stroke? Thanks!
1998 jeep zj 4.0L AW4

97grand4.0

Have been similar stories on here where the intake and exhaust manifolds were either the wrong part, or incorrectly installed causing vacuum leaks and or the injectors got switched around in the process. Of course you should use a scan tool to check for operation of the obd2 stuff.

SirCyber

Can you rent those from AutoZone? It's not running well enough to drive anywhere

97grand4.0

you can buy them for about 120 bucks, but I recommend ebay for like 40 or less for a good actron with graphing. You can then watch the tps waveform and see if it drops out.
If his woes began when he replaced the manifold I would instead be looking there. If not then anything is possible. Check fuel pressure.

SirCyber

Only these woes. It ran rough, but was identified as an exhaust leak, caused by the factory exhaust manifold being very badly cracked.

dave1123

The 98 was still a one piece steel tube manifold, but remember in order to replace it, the intake manifold had to be removed at the same time. Have you tried switching the 1&2 injector plugs? If I remember correctly the gasket and exhaust manifold locates on 2 dowels so it should be aligned right, but the intake could be shifted and causing a vacuum leak. Also, check that you don't have the spark plug wires connected wrong.

It's the 99 and newer that has 2 cast iron manifolds and your dad's ZJ also has the 42re transmission, BTW. The AW4 was only used in the XJs.

It's entirely possible something on the intake side got damaged when it was pulled. Are all the vacuum lines connected? Even a cracked rubber connector can make it run rough. I think there's a pair on the side of the manifold that go to the purge pump and carbon canister. On my 2000, the MAP sensor is mounted at the base of the throttle body and can get damaged very easily when the manifold is removed.

The injectors are controlled by the PCM using the crank and cam sensors for timing.

SirCyber

dave1123, excellent info! See, tons of ignorance on my behalf. So, from the sounds of it, check vacuum hoses, make sure intake manifold is seated where it's supposed to be. Dang I bet that last part is the problem! Check MAP sensor located at base of throttle body.
i did try to swap #1 and #2 injector plugs, #2 wouldn't stretch to #1, so my assumption is that they are right where they belong. I suspected that the gasket was where it belonged, then, in a flash of brilliant ignorance, thought that maybe the gasket was on backwards... With the pin alignment, as you told it, that would be erroneous on my part.

Thank you for places to look, I will forward over this advice on where to look.

Question, before I lose it. How do I check the intake manifold to see if it's not on right? All else I know how to check, but that one I haven't done before. Also, I looked around with my phone flashlight, but could not see vacuum lines to intake... Can you tell/show me where? Mind you this was the first time he tried to start it with me nearby, and the youngest jeep I have owned was a '95, my current is a '90 and my first was a '93, all XJ's. Not familiar at all with the Grands

97grand4.0

I mean, my first thought was an intake leak, but now I think a vacuum leak would make the engine rpm increase, not decrease!
Before I went through that I would make sure everything else was ok. Like for instance the front O2 sensor, that would cause the original syymptoms..which predated the manifold change...but you would think that would set a check engine light. Even a spark plug wire that was burnt can be hard to find and cause havoc.

97grand4.0

"About 3 months ago, dad had a friend replace his exhaust manifold due to some really gnarly cracks in old one. Dad says it's a Borla off of Amazon. Ever since, it has run like doggy doo. I got to hear it, and this is my observation."

dave1123

You can check for a vacuum leak using anything flammable, like brake cleaner or TB cleaner, but what I like is a propane torch. WAIT! not lit! just crack open the valve and play the gas around suspected areas. The engine will speed up when it sucks the gas in. It's clean, dry, and doesn't make a mess. Even if a spark sets it off, it'll just be a flash.

The vacuum taps are on the bottom of this pic. I doubt the manifold is not lined up, but it could be. This pic is of a 99-04 manifold, but yours is similar. I'm stupid and don't know how to draw circles and arrows on a photo. They are on the side rather than the top. My 97 ZJ was too long ago for me the remember if it really had those. My WJ does and they go to the canister purge pump and leak detection pump.

Now that I think about it, your MAP sensor is mounted on the manifold and connected to the throttle body by a hose if I'm not mistaken. Check that short piece of hose for cracks.

SirCyber

Ahh, I didn't explain the runs like crap... Sorry... It's running super rich, can smell the gas. The backfire I mention happened at startup, and, belched a gout of flame poof from front of engine. Then it ran really rough, almost like it was out of timing. Also very low RPMs. Gas was sluggish to respond. After that, while I was there, it kinda ran once, very very low RPMs and dogged, and then sputtering, died.

dave1123

Alright, just for grins, disconnect the cat and see if the problem disappears! It could be plugged and was getting worse over time. My wife's K-car actually blew the EGR valve off the engine before I realized what was going on. Separate the pipe at the header and see if it starts and runs good. Don't run it long that way or risk setting the jeep on fire! If it's easier to pull the cat itself, do that. You can cut the pipe and weld it after if that's not the problem.

With the Dodge, I tried to blow thru the cat with my mouth and got nothing!

SirCyber

It ran better before the exhaust manifold was installed, so I don't think that's the problem, but I will send that info to him. I'm in KS for vacation w/ family so have to forward on info to him in wa

dave1123

Tell him to read this discussion on the website. It may be more help that a simple phone call.

SirCyber

Hey guys, reviving this thread with an update. I now have the zj in my possession, just replaced the cap and plugs that my dad kept, and soon to be finding a propane torch to start the testing! Will take video of it running and post link as soon as it is available.