So...it's about to go to the dealer

[BIGSLVRXJ]

Alright ya'll, here's what I've got. 1990 XJ 4.0 AW4. I posted a while back in mod tech because I have an HO intake on an 87 motor. However I don't really think this belongs in mod tech anymore because I have pretty much figured out it has nothing to do with the HO intake. About 2 months ago I got done putting a "new to me" motor in my 90. It was a 124k mile young 4.0 from an 87. I bought the APN header thinking it would work, however it wouldn't clear my Renix intake manifold, so I used an HO intake manifold. I used the correct mixture of sensors from the Renix/HO years to make it work just fine. The problem I'm having is that in the morning when I first start it up it will idle fine for about a minute, around 700-800rpms about where an XJ should. Once it gets going for about a minute however it will drop significantly, to around 250-300rpms. I'm also having an issue with it backfiring and bucking in the morning but then it will go away for the most part and it has plenty of power. Fuel pressure is more than adequate resting at about 33psi with the regulator line on and about 42 with it off. Mechanics stethoscope(sp?) tells me all the injectors are firing just fine as well and by the way they are the Renix injectors. EVERY sensor aside from maybe the SYNC has been replaced. Throttle body is clean and so is the IAC passage. I loosened the distributor clamp and tried moving it around in the groove it was resting in, it seemed to help slightly with the backfiring but none with the idle. The idle screw on the HO throttle body has not been moved either since right now it is set how every other HO I've seen is. I'm honestly thinking about taking it to the dealer and having them hook the ignition scope up to it to check out the whole slew of things but first I figured I'd prod at you guys once more. Can anyone help?

 

[XJING]

EGR, take it out of the mix. Temporaraly plug the vacumn line and make sure that the valve on the manifold is closed all the way. Unplug the battery for a bit,(to reset computer) and then fire it up and see if it helps. Its the only sensor I can think of that could do what its doing to you. YMMV, Good Luck

 

[Wa Woody]

I know it would be a PITA, but have you thought about putting the old Renix intake back on to see if problem is in the HO intake or associated sensors. If you didn't want to go through all of that, I wouldn't either; I would retest all the sensors, check for any air leaks on the manifold, and check the 101 connector one last time then take it to the dealer. With that many miles I would also think about new injectors, even if they aren't the problem they are due. BTW a Borla header fits the Renix manifold, that's the one I'm running, and may have saved you a lot of frustration. Good Luck

Woody

 

[BIGSLVRXJ]

EGR, take it out of the mix. Temporaraly plug the vacumn line and make sure that the valve on the manifold is closed all the way. Unplug the battery for a bit,(to reset computer) and then fire it up and see if it helps. Its the only sensor I can think of that could do what its doing to you. YMMV, Good Luck

The whole EGR system is gone with the HO intake, I've tried just plugging the solenoid back in and that doesn't affect it at all.

I know it would be a PITA, but have you thought about putting the old Renix intake back on to see if problem is in the HO intake or associated sensors. If you didn't want to go through all of that, I wouldn't either; I would retest all the sensors, check for any air leaks on the manifold, and check the 101 connector one last time then take it to the dealer. With that many miles I would also think about new injectors, even if they aren't the problem they are due. BTW a Borla header fits the Renix manifold, that's the one I'm running, and may have saved you a lot of frustration. Good Luck

Woody

See the only problem with that is I don't have a Renix exhaust manifold anymore, mine was cracked which is why I went with the header deal. I'm almost positive I have no vacuum leaks because that usually causes a higher idle and I know all the ports are plugged and my lines are brand new. I also don't have the C101 connector because IIRC in 90 Renixs they did away with that. I will consider the Borla if it comes back to me absolutely having to put the Renix intake back on but I really don't think that's my culprit here. Too many people have done exactly what I did successfully to have that be the case. Thanks for the help so far guys. Anyone else?

 

[90xj06]

i 2nd the egr thought. when it warms up the gasses expand and cause a binding.

 

[BIGSLVRXJ]

i 2nd the egr thought. when it warms up the gasses expand and cause a binding.

Yeah I just don't think it needs it though, the HO engines run just fine without the EGR. Hell I had mine disabled in my Renix manifolds for a while and it ran ok.

 

[BIGSLVRXJ]

Well I just called 2 of the 3 dealers in town, two refused to even look at it because they thought it would be too time consuming. The other one said it's $90 just to hook it up to the ignition scope. If anyone else has any suggestions I'm open to anything here.

 

[BIGSLVRXJ]

I hate to be stingy and bumping this but I still really need help. Would a MAT or CTS be screwing up the engine that quick if they were reading wrong? Like I said I did just replace all the sensors but I suppose one could still be bad.

 

[XJING]

Man, this is very strange, I can't even hazard another guess, but since it runs fine upon start up. then rpms go down, I am thinking maybe something to do with the IAC moving (I know everyone blames everything on CPS, IAC or TPS)
Go with me on this for a minute, the iac is at the right position via the latch relay when you shut off the Xj, then you fire it up and it runs well. and then the IAC might just move and give you the result of low idle, till you are warmed up and its in its "right" position. I am not too up to speed on the electric side of the IAC but thats my next guess. Well take it for what its worth. Good luck

 

[BIGSLVRXJ]

Man, this is very strange, I can't even hazard another guess, but since it runs fine upon start up. then rpms go down, I am thinking maybe something to do with the IAC moving (I know everyone blames everything on CPS, IAC or TPS)
Go with me on this for a minute, the iac is at the right position via the latch relay when you shut off the Xj, then you fire it up and it runs well. and then the IAC might just move and give you the result of low idle, till you are warmed up and its in its "right" position. I am not too up to speed on the electric side of the IAC but thats my next guess. Well take it for what its worth. Good luck

Thanks for the quick response man. Yeah I'm almost wondering if it does have something to do with the IAC. I replaced the latch relay as well, but it didn't do anything. I've got the aftermarket Borg-Warner IAC motor that has a lifetime warranty but I just can't test if it is bad or not.

 

[90xj06]

what about air temp sensor?

 

[BIGSLVRXJ]

what about air temp sensor?

Is that the same as the MAT sensor(manifold air temp)?

 

[XJING]

Is that the same as the MAT sensor(manifold air temp)?

yes, it is and I dont think thats your problem for alot of reasons.

 

[BIGSLVRXJ]

yes, it is and I dont think thats your problem for alot of reasons.

Well see that just got replaced too. I don't really think that will affect idle much either. I'm just really starting to wonder if it is the CTS. Because as the coolant warms up I think it changes the idle around a little bit. But again...brand new sensor so that doesn't make sense(no pun intended).

 

[XJING]

Thanks for the quick response man. Yeah I'm almost wondering if it does have something to do with the IAC. I replaced the latch relay as well, but it didn't do anything. I've got the aftermarket Borg-Warner IAC motor that has a lifetime warranty but I just can't test if it is bad or not.

give it a good cleaning and check the insides of where it goes in the TB to make sure there is nothing it can get caught up on. then if that doesn't do it get another IAC, since its on lifetime warranty.
Does it smell at all when it drops idle, like maybe runnin rich? just trying to get alittle more info to diagnose. Good Luck

 

[BIGSLVRXJ]

give it a good cleaning and check the insides of where it goes in the TB to make sure there is nothing it can get caught up on. then if that doesn't do it get another IAC, since its on lifetime warranty.
Does it smell at all when it drops idle, like maybe runnin rich? just trying to get alittle more info to diagnose. Good Luck

No it doesn't smell then but like when it backfires it smells like raw fuel bad, out the exhaust pipe. I've checked fuel pressure at the rail at idle and it is in the high area of where it should be, so it's not like it is too high.

 

[XJING]

no I dont think its the CTS, CTS=nothing to do with idle.

 

[BIGSLVRXJ]

no I dont think its the CTS, CTS=nothing to do with idle.

Oh ok, good to know. Any other thoughts? I'm willing to give anything a shot, I will definitely mess with that IAC tomorrow some. Although it should still be clean I'll check the passage and such.

 

[90xj06]

im going to say either sticking fuel injector.... or map.

 

[BIGSLVRXJ]

im going to say either sticking fuel injector.... or map.

It could be a sticking injector, but the MAP was just replaced as well. I've tried doing my best to test the injectors but that still may be it.