Expat_PM

replaced my 88 4.0 engine (with a used 91 4.0). Put my intake and parts on the 91. Put on a new fuel pump and new front and rear Vacuum harness. Backfired like CRAZY when I connected the MAP to vacuum. Old MAP looked like it was maybe cracked so I put on a new MAP.

Still backfires like mad.

Runs kinda crappy but gets okay mileage with MAP disconnected. But I gotta figure out what's up with the MAP causing backfires (exhaust backfires).

Any ideas?

Jeff

DFlintstone

The MAP aside, which flywheel or torque plate did you use? 91 is different than 87-90, might do odd things like backfire. Don't know there for sure, just that the gaps that the CPS reads are different, might cause spark at the wrong time?

Expat_PM

Good point DFlintstone,
I used my original flywheel.
Basically I did Cruisers HO Swap (even though it was a 91).
I just used it like a long block and swapped every single thing from my original engine over to the new block (long block).

I have a new fuel pump. The old one was mush as was the fuel filler hoses. New fuel lines (might as well, it was relatively cheap) and filter. New Vacuum Harness (both halves) and new MAP.

Also, while I had the motor out a tweaked the valve cover tubes on the inside so it would not throw so much oil into the air box. New tune up stuff all around (wires, plugs, etc) too.

Right now I think I have all vacuum lines connect (even have them connected to the right stuff) except the MAP sensor. When I connect that it backfires like crazy). My old muffler and tailpipe were ancient and I had not gotten around to changing them out but this morning when I connected the new map it backfired so hard it blew the tail pipe off (separated from the muffler).

Runs so-so as long as the MAP vacuum is not connected.

DFlintstone

Here, fourth link down is "sensor diagnostics" has a rough map test. https://www.cherokeeforum.com/f51/he...ix-links-1397/ I guess to get fancy you use an analogue meter, (or a spendy digital), and a hand vacuum pump and actually watch for a smooth sweep. The map telling the ECU to dump too much fuel might blow a muffler off I suppose.

Have you checked your grounds with a meter? You want close to an ohm from the sensor ground circuit, (including at the MAP), to engine to bat to body....

Also dealing with that C-101 is a thought before getting too tangled up in dead ends. I mean If I had one I'd likely bypass and solder....if I had the time, and this year, the weather!

Expat_PM

I refreshed all the grounds under the hood and had planned to eliminate the C101 today but instead I've spent it tinkering trying to figure out this MAP and replacing the exhaust (LOL). It was awesome, I was in small town rural Alabama driving right by the police station when it blew and it was like time stopped for a few seconds as the whole town looked at me and just froze for a second.

I didn't bother testing the MAP I had since it looked cracked and I put on a new one. I normally wouldn't just throw parts at a problem but it looked damaged so seemed like a pretty likely problem.

I haven't metered anything under the hood yet. Plan to as soon as I get the AC blower motor in my house blowing again. Wife just told me it was "eight hundred degrees" upstairs and the blower seems not to be turning.

Heck, it's only April and will get a whole lot hotter than this in a few months.

DFlintstone

Guess I'm about stumped. If it can stop time in Alabama does seem like being maybe too much fuel being MAP related. (isn't it a bit slow there already?) Mine normally dies with the MAP vacuum unplugged. Injectors? The stock are two-piece black with metal. (crap Simmons Decca or something that leaks eventually). Over rich replacements might be solid one piece plastic. At some point you might want to check those #'s if that's the case, I have the flow tables. Sun here today for a welcome change! More rain forecast tomorrow.

cruiser54

Make sure your dizzy is indexed correctly. Tip 13.

Did you use the intake gasket for the Renix intake?

freegdr

Also check for cracked vacuum line to map just had same issue on a Renix right at throttle body . Morning Major.

cruiser54

Morning General.

Expat_PM

Wow, so much great feedback from the champions of CF!
Let's see, where to begin?

Thanks DFlintstone. Ordered a set of replacement injectors (746s). They should be here tomorrow and that's an easy upgrade. Maybe I do have leaky/cracks injector(s) causing problems. We'll see when the Bosches get installed.

Cruiser, Have not done #13 yet. Still working through the first 5. BTW, what a great write up you've done. I'm sure I speak for every Renix owner when I say thanks a million.
I did use the Renix intake gasket and inspected it very well before putting it on to make sure it was still hunky-dory.

freegdr, This beast had totally hosed up (pun intended) vacuum lines and I just bought the new vacuum harness for it (both halves) and started from scratch. But being the little plastic ones do break somewhat easily I replaced the one from the Throttle body to the Map with a good quality rubber line from Napa. No change.

I think what puzzles me the most is that it seems to be completely backward: it runs okay (not great but okay) without the MAP and backfires, sputters and carries on with the MAP. Go figure.

Thanks again everyone and I hope you all have a wonderful Easter.
He is risen.

DFlintstone

Don't know about "Champ", but I was the "guinea pig", for the 746's. I'm sure you will like them. > https://www.cherokeeforum.com/f2/199...r-swap-163105/

ajpulley

Yes, it is backward for a normally running engine.

Thinking out loud. Perhaps, the condition you are experiencing while the MAP is disconnected is a similar condition on a normally running engine when the MAP is connected... Maybe the fuel ratio is close enough when not connected, then is too rich (or lean) when connected? If the MAP is giving close to accurate readings, then there is an external issue that is only reacting to the ECU's reaction to the MAP. Still sounds as if timing is slightly off.

Expat_PM

Thanks AJ,
This is all a lot to think about.
Sounds like I need to Cruiser's advice and index the dizzy - imagine that, Cruiser may be right. Lol

djgrayxj

Note from DJ
I just looked over all of the posts regarding your problem and one statement by you left me confused.
You listed all the new items you have installed regarding this change out.
But in your post #10 you stated the following [I did use the Renix intake gasket and inspected it very well before putting it on to make sure it was still hunky-dory.] are you saying you reused the old gasket that hand the imprint of the 88 head against the 91 head.

cruiser54

Nice catch.

I just edited the Tip on my website to clarify using a new gasket.