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.

[90xj06]

well true but there is nothing to say that i could have failed.

[XJING]

I once had a crank position sensor that the tip on it was FUBAR and it was hanging on be a little piece of metal it would idle fine then do all sorts of strange things with my idle, but I dont think thats your problem. I am not leaning toward the MAT idea because of the way RENIX works. I am thinking its the electric side of the IAC. good luck

[BIGSLVRXJ]

I once had a crank position sensor that the tip on it was FUBAR and it was hanging on be a little piece of metal it would idle fine then do all sorts of strange things with my idle, but I dont think thats your problem. I am not leaning toward the MAT idea because of the way RENIX works. I am thinking its the electric side of the IAC. good luck
Again not saying I don't have sensors that couldn't have been bad out of the box, but I just replaced the CPS also. I will definitely screw with that IAC tomorrow. It also seems to be doing the "stick" up on start up where it revs to like 1500-2000 rpms and then settles back down. So you may be on to something there. I'll report back after tomorrow.

[90xj06]

just a question but does the iac pintel completely go in after it warms up?

[90xj06]

what if you where to remove the iac and use a screw driver to simulate the iac?

[XJING]

http://www.olypen.com/craigh/idle.htm
When I am all out of guesses I usually look here, might try this guys fix, all his mods and fixes have always worked for me. good luck

[BIGSLVRXJ]

just a question but does the iac pintel completely go in after it warms up?
I guess I'm not too sure on that.

what if you where to remove the iac and use a screw driver to simulate the iac?
I don't think that would work because there would be a huge vacuum leak wouldn't there?

[XJING]

what if you where to remove the iac and use a screw driver to simulate the iac?
thats pretty funny, have you ever taken apart the IAC, first off you would probably scratch the inside of the run with a screw driver, the IAC is a relativly persision instrument and you cant replicate what it does with a screw driver, thanks for playing....

[Mechanic_Dave]

I don't know what your problem might be, but i would gather its something with the idle circut as well. I feel for you, I have had more then my fair share of electrical problems myself. If you still can't figure it out.. megasquirt her. Look it up, lotsa info, support and adds more programability. IE do it yourself FI.
If you want to rule out the injectors though, take them in. I took 4 in last year, had them tested, and one rebuilt. Cost me really minimal (and I am cheap when it comes to spending money on mechanics). Just try to find a local shop that specializes. My only recomendation, don't listen to the shop, replace the o-rings while they are out. :p It will save you a lot of stress.

[BIGSLVRXJ]

If you want to rule out the injectors though, take them in. I took 4 in last year, had them tested, and one rebuilt. Cost me really minimal (and I am cheap when it comes to spending money on mechanics). Just try to find a local shop that specializes. My only recomendation, don't listen to the shop, replace the o-rings while they are out. :p It will save you a lot of stress.
Yeah I may do that, I remember calling one place about a year ago and they said they can't test them without having them in the vehicle or they'll burn them up or something (which I don't believe) but I think I'll call around some more. Oh and yeah o-rings have already been replaced, it only takes 3 of them leaking at once to finally make me decide I should change the orings out.

[letank]

noobie to XJ here, but not to heeps.... i have a 74 wagoneer.... and a 90 wrangler.... looking to increase the family....

I always look for the simplest and cheapest things first.

Good grounds.... battery to engine engine back to frame (or is it the reverse for the XJ). even externally good cables can be internally corroded.

How much voltage is your alternator giving. After a minute, this is when full voltage kicks in. Measure at battery and in circuits that are fed DC voltage.

you mentioned backfiring: usually means timing is off or too much raw gas in the exhaust

let us know

Michel
74 wag
90 wranwran

[BIGSLVRXJ]

noobie to XJ here, but not to heeps.... i have a 74 wagoneer.... and a 90 wrangler.... looking to increase the family....

I always look for the simplest and cheapest things first.

Good grounds.... battery to engine engine back to frame (or is it the reverse for the XJ). even externally good cables can be internally corroded.

How much voltage is your alternator giving. After a minute, this is when full voltage kicks in. Measure at battery and in circuits that are fed DC voltage.

you mentioned backfiring: usually means timing is off or too much raw gas in the exhaust

let us know

Michel
74 wag
90 wranwran
My grounds are all spotless. In fact I replaced the ground braid and all battery cables with a custom set of 5-90s 2/0 gauge or whatever and all are clean, so I'm relatively sure that's not it. I'll check the alternator output. Yeah backfiring, that's what I would think too but it's not getting too much gas. I've played with the distributor some just moving it around in the groove that it is in and nothing really changes. At this point I'm just trying to figure out if the low idle and backfire are related or if I have 2 different problems.

[BIGSLVRXJ]

Just thought I'd let all you guys know that helped, especially XJING, that is was the IAC motor. It actually wasn't bad but I was using the HO one when I should have been using the Renix one. Dur. Anyway thanks again guys, that saved me some serious cash I'm sure.I don't think I ever would've thought twice about the IAC again.

[XJING]

Glad your back up and runnin right.