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Renix backfiring with HO intake

BIGSLVRXJ

NAXJA Forum User
Location
Des Moines, Iowa
Hey guys I just got my HO intake and APN exhaust manifold installed on my 90. The thing runs like a top for the most part, idle is great, sounds great running, all that. But when I accelerate it backfires really bad. At first I had the exhaust hanging off the headers so I thought maybe having absolutely NO backpressure was causing it. It seems to have the power definitely but its now mainly around 35 or I suppose the 2000 or so rpm band that it does it, right as its about to shift to OD. My question is what could be causing this. Backfiring is the result of the inproper ratio of fuel/air right? I used the 90's fuel injectors, I used the 90's TPS (and it is adjusted to .82% or so), and I used the 90's IAC, those are the only sensors I could imagine that would effect it. Oh yes, and I also used the 90's 02 sensor. Anything else you guys can think of?
Thanks in advance,
Collin
 
I just did a similar swap and I ended up using the IAC from the HO. The renix IAC did not have a long enough plunger when used on the HO throttle body. It runs great - perhaps this could cause some of the issues you are having.
 
That is usually caused by a lean condition.

The IAC will have no effect on this, as the throttle is open, and the engine is getting the majority of its airflow by the throttle blade.
 
jeepownzme said:
I just did a similar swap and I ended up using the IAC from the HO. The renix IAC did not have a long enough plunger when used on the HO throttle body. It runs great - perhaps this could cause some of the issues you are having.
I started out by using the HO IAC. I thought it idled rougher and ran crappier with that. So I threw my Renix one back on and it smoothed the idle out. Interesting thing I noticed was when I put the HO one on, it got rid of the common Renix "rev up" at start up. I don't know, just for fun I may put the HO one back on there and see if it quits backfiring. Thanks for your input.
dave2 said:
That is usually caused by a lean condition.

The IAC will have no effect on this, as the throttle is open, and the engine is getting the majority of its airflow by the throttle blade.
That's what I thought but what doesn't make sense is a lot of people use HO injectors and they find it's running too lean; however I started out with Renix injectors and it's still backfiring. The only thing I can think of is maybe when I had my 02 sensor sitting on the ground for two months while this debaucle was going on it somehow got fouled. But I have no idea. Thanks for your help too.
Any other stabs at this from anyone?
 
Last edited:
timing issue? I could see a distributor one tooth off casing something like that. Have you put a timing light to it?
 
check your MAP sensor. that's what sends all the info to your computer about your fuel/air ratio so maybe sure thats working right.

I was going through hell a couple weeks ago about my Jeep backfiring and whatnot and it ended up being a few things but the main one was the MAP and I replaced it and it runs like a champ now
 
muduck18 said:
back fire on or off the throttle?
Usually on, however I found out today after I drove the thing some more that it only does it until it's warmed up then it goes away. I forgot to also add that I have no cat, it comes off the headers into a straight pipe and then a Flowmaster 40. I hate to ask too, but what is a timing light? I understand about the indexing of the distributor but is a timing light somehow you measure that or what? Sorry for the lack of knowledge here, I've just never really screwed with it.
TIA guys,
Collin
 
BigBlackHeep said:
Usually on, however I found out today after I drove the thing some more that it only does it until it's warmed up then it goes away. I forgot to also add that I have no cat, it comes off the headers into a straight pipe and then a Flowmaster 40. I hate to ask too, but what is a timing light? I understand about the indexing of the distributor but is a timing light somehow you measure that or what? Sorry for the lack of knowledge here, I've just never really screwed with it.

sounds like the O2 sensor. does it call for two or just one?
If it only does it when it is cold it I would guess that its the 02.
but just for s+g's check your CPS.
if you have no cat then do you have enough back pressure still? the HO likes a fair amount of back pressure. try adding a muffalar to the end of that straight pipe or a size down/tight curve on the end of the pipe see if it makes a differnce.

Timing light... google it...
its a light gun that is used to show when piston should be at tdc and if it is on the right notch in the distributer.

is this engine a complete rebuild or did you just add the ho intake and ex manifolds' ?
if you have not taken the distributor or timing chain off I would not worry about it.
it sounds more like a sensor.
 
muduck18 said:
sounds like the O2 sensor. does it call for two or just one?
If it only does it when it is cold it I would guess that its the 02.
but just for s+g's check your CPS.
if you have no cat then do you have enough back pressure still? the HO likes a fair amount of back pressure. try adding a muffalar to the end of that straight pipe or a size down/tight curve on the end of the pipe see if it makes a differnce.

Timing light... google it...
its a light gun that is used to show when piston should be at tdc and if it is on the right notch in the distributer.

is this engine a complete rebuild or did you just add the ho intake and ex manifolds' ?
if you have not taken the distributor or timing chain off I would not worry about it.
it sounds more like a sensor.
I'm really starting to lean more towards 02 sensor or maybe IAC. I thought that was the deal where if you didn't take distributor or timing chain off you didn't have to mess with indexing which is good. This swap started because I was putting a "new to me" engine in my Jeep and I ordered the APN header and my Renix intake wouldn't fit over the APN header so I had to use an HO one. I didn't remove the dist. or timing chain from the other engine so I don't think it would be a timing issue unless it has to do with the CPS somehow. I had no cat with my last motor and it was fine backpressure wise so this one should be too. This motor is from an 87 and my Jeep is a 90 by the way. Again thanks everyone for the help so far and if anyone else has suggestions those'd be great too.
-Collin
 
are you using the HO intake on a Renix head? if so your restriction the airflow bigtime to the head. back in the day I did the opposite by putting a renix intake on a HO head in a 90XJ and had to get a specially modified renix intake manifold to get the ports to line up correctly
here is a pic of the intake differences:
the modified renix is top and the stock renix is bottom
IMG_1459.jpg


the renix intake manifold runners come to the head at a uppward angle and the HO intake manifold runners come to the head at a downward angle.
if in your diganosis of this problem you pull it back off take a look your self and see the differences between the Renix and HO intakes.
just another idea on the problem
 
I have a 1990 with a 95 HO 4.0L motor in my XJ. Used the 1990 intake and checked the runner alignment...95% of the runner is aligned. the intake mounts just fine to the HO head. Given the higher intake runners on the HO head, it moves the intake up about half an inch.

The 5% of the runner that was blocked...Probably was blocked by a HO intake as well. Without porting the head and intake to match, you would expect this from factory parts.
 
Bouncy said:
I have a 1990 with a 95 HO 4.0L motor in my XJ. Used the 1990 intake and checked the runner alignment...95% of the runner is aligned. the intake mounts just fine to the HO head. Given the higher intake runners on the HO head, it moves the intake up about half an inch.

The 5% of the runner that was blocked...Probably was blocked by a HO intake as well. Without porting the head and intake to match, you would expect this from factory parts.
Yeah I did a bunch of checking around first, I held the gaskets side by side, looked at the intake side by side and like you said, its about 5% blockage if that, its certainly not major restriction. Again if anything I'm still convinced the thing is breathing better with the bigger throttle body. Anyone else have suggestions?
 
Ok I've got another update here. I went back to the HO IAC and it made no difference whatsoever. However while I was tinkering I noticed a few things. The idle stays about where it should but you can hear it fluctuate. It will go from smooth to rough and then back to smooth and rough. It will just jump like 100 rpms about every 20 seconds at idle and smooth itself out but it keeps going back to rough. Also I noticed if I pull the vacuum line off the MAP sensor while it's running it kills it right away. Is this normal? The TPS should still be good since its about 4 months old, that's the only other thing I could think of. Anyone else?
 
Tps is working stepper is working Map sounds like it is working. Yes it is normal to kill the engine when the vac is pulled from the MAP.
what about your cps?

If all your sensors check out. figure out how to measure the vacume levels for your vacume system. maybe the HO intake has a differnt vacume levels. it still sounds like there is a vacume line off or missing or a sensor.

good luck.
 
muduck18 said:
Tps is working stepper is working Map sounds like it is working. Yes it is normal to kill the engine when the vac is pulled from the MAP.
what about your cps?

If all your sensors check out. figure out how to measure the vacume levels for your vacume system. maybe the HO intake has a differnt vacume levels. it still sounds like there is a vacume line off or missing or a sensor.

good luck.
I will try to test the CPS later today. I know for a fact I don't have a vac line off and I don't think I'm missing a sensor but I'm thinking I may have a bad sensor somewhere.
 
'Nother update here. Recently the thing has been idling fine in park in the morning and dropping to like 200rpms (according to the tach) when it's in drive and warmed up. Today I replaced the IAC (just for fun) and it made no difference. I'm still going to check the CPS tomorrow, and I'll make sure my TPS stayed in adjustment as well. I'm sort of leaning off the timing issue since I didn't remove the distributor or timing chain but I'm willing to try anything. Any other suggestions? I really appreciate all the responses so far.
 
Ok, this is the last thing I got to thinking. I relocated my Coolant Temperature Sensor from the original spot right by the exhaust manifold to the thermostat housing. I know the temperature probably doesn't stay very constant up in the thermostat housing, like it did further down. Could this have anything to do with it? Maybe sending faulty readings to the ECU? Last thought of the night before bed, probably just talking out my ass.
 
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