MPG spark plugs plug GAP & Renix

This question and discussion is based on a post from another thread where someone replaced their worn 0.043 gap plugs in a '90 Renix engine with new .035 gaped plugs, after which the mileage dropped by 2 mpgs from 19 to 17.

So my question is:

Has anyone experimented with, and or experienced changes, good or bad with different plug gaps on the Renix Era engines? If so has anyone found a more optimal plug gap, or plug and plug gap to maximize mileage. IF so what were the differences in mileage, the gaps and plugs used!

This question might need to include discussion of the old MTBE and new E10 ethanol gas as well used in the tests.

I see no reason not to include the HO motors, and OBD-I and OBD-II years, but please specify the year and any non OEM add ons if any, as the 3 different computers and the HO engine might like different gaps and plugs than the Renix, etc.

I am currently using an autolight platinum single electrode plug at a 0.035 gap when it was installed about 10,000 miles ago and getting about 20 mpg on a pure 55 mph highway run (call it non stop) now, and averaging 14.5 mpg with mixed city driving on my 87 Renix 4x4 all stock XJ, and stock 3.55 rear end. As I recall I checked my gap earlate last year, and it was still 0.035 unchanged.

So let the Mayhem begin!!!! Post your data and results here!!!!

Not to change the subject, but maybe add alittle to it. Back in my hot rodding, small block chevy days, we used to open up the spark plug gap a bit, but with a higher output coil. I really don't see any benifits in narrowing the gap.

I don’t know that would increase mileage for sure, I have always run my plugs at .40. Now I run Plats and Iridium’s and don’t have any erosion probs and they require much les power to spark but if using std cheapie plugs you may want to stay at .35 as they will burn away very fast say 10-20k they may be close to .40 and require much more power.
I get about 17mpg daily 19-20 on hwy trips in my MJ and its lifted 6" with 33" and 4.10 gears, The XJ only gets best of 15 daily 17 hwy but its stock 3.55 gears

Comanchedude said:

I don’t know that would increase mileage for sure, I have always run my plugs at .40. Now I run Plats and Iridium’s and don’t have any erosion probs and they require much les power to spark but if using std cheapie plugs you may want to stay at .35 as they will burn away very fast say 10-20k they may be close to .40 and require much more power.
I get about 17mpg daily 19-20 on hwy trips in my MJ and its lifted 6" with 33" and 4.10 gears, The XJ only gets best of 15 daily 17 hwy but its stock 3.55 gears

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I was thinking the same thing, that the plat/irridium plugs would be the better choice for wide gap trials since they do not errod like standard electrodes do, they would hold the wide gap longer. So do you think the 3.55 gearing is holding your mileage back?

ia green96 said:

Not to change the subject, but maybe add alittle to it. Back in my hot rodding, small block chevy days, we used to open up the spark plug gap a bit, but with a higher output coil. I really don't see any benifits in narrowing the gap.

Click to expand...

Yeah, thats a good way to make higher rpm power.

Ecomike said:

I was thinking the same thing, that the plat/irridium plugs would be the better choice for wide gap trials since they do not errod like standard electrodes do, they would hold the wide gap longer. So do you think the 3.55 gearing is holding your mileage back?

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I guess? it runs really well and and the prior owner replaced the injectors,egr,aic,tps, so it has many new parts but don't know if the Ox sensor was replaced/ may just be what that combo gets. I don't have a tach on it but feels like the rpms are higher than my MJ so could just be gearing.

So if we have low gearing, like 3.55:1, then bigger tires might help us with gas mileage up to some ??? tire size, where as someone with a lot higher rear end differential gear ratio might start loosing mileage if their tires get too big?

I know there are a number of other threads here on the gearing and tire size ratio, but I never really locked in on the details in them as I just figured out what gear ratio my rear end has this past week. Guess its time for us to re read those older threads.

So back to the original topic, has anyone else tested various spark plug gaps on the Renix 4.0 with the aim of improving gas mileage, if so what worked best???

This question also relates to the new gas formulas using ethanol which we have all noticed to be decreasing our mileage and with respect to the fact that ethanol burns differently from real gasoline!!!! Once again I got this idea from 2 sources, one is my sons 96 Ford taurus uses .044 gap stock, versus the Jeep standard of a 0.035 gap, and from a guy who posted a thread here a few days ago saying he installed new spark plugs, reducing his plug gap from 0.043 (used plugs) to 0.035 only to see his gas mileage drop off by 2 mpgs on his 4.0 Renix Jeep.

Where are those ENGINE GURUS when we need them????

This question has been going through my mind also. I just changed my plugs last weekend Champ OEM. Boy were they old. The gaps were .065 - .075 new ones at .035. I wondered if they were original. 2000 XJ Classic 79,000 miles. The XJ ran fine before and after. MPG before 18.7 I will report after I get a good reading.

I'm currently running Bosch Plat. Plus plugs, gapped at a hair over .035. Last good MPG test I got (before the o2 sensor and others let go over the winter) had me at a 6 month average of 19.89 MPG. (for my 1989 XJ )

My next investment is a set of Champion Copper Plus, will be gapped at .40-.045. Haven't decided yet.

Dump the Bosch..they have issues w/the center electrode over heating and melting out,effectively increasing the gap. They also have a tendancy to cause minor misfires that, although you can't feel them, will reduce fuel economy. it gets worse with their multi-electrode designs.(those aren't gappable and preset to .090")

in my opinion the wider the gap the better, as long as the ignition system can reliably support it w/ a strong spark....also worth a mention are multispark systems.

I run champion coppers gapped to the fac. spec. (.035"?...I don't remember I always use the label...lol)in my 97 4.0L w/16mpg city

'90 XJ, 4.0, 3.55, AW4. Bosch seemed fine until the center electrode disappeared after 15,000, so I switched to plain old NGK gapped .035. I see no diffference, about 14 around town, and I checked an interstate run averaging 55mph with 19.8. That average was miles traveled vs. door to door time, so ok, I was doing about 78 on the open road. No complaint there.

Renix ignition systems are not high performance, other that reduced erosion, I don't see much advantage in expensive plugs. It's my belief the makers moved that direction largely to reduce misfire codes on OBD systems - my son's '99 doesn't get any better mileage than I do.

Multispark capacitive discharge systems sound great for the high performace, but 1) don't see the makers using them much 2) the multispark tapers off after 3000 rpm, right when you really need it 3) somebody please show me a clear before and after dyno run that demonstrates exactly what can be gained - most magazines avoid it for some unspoken reason . . .