Hi Guy's,

I'm having problems with my 91 3.1 V6 at idle, I'm currently looking for whats wrong but I decided to change the plugs as part of my investigations, so sent for some Denso U groove.

The problem is, I've taken out Champion RV15YC4 plugs, but the Denso T16EPR-U plugs have a longer thread if measured from the tapered seat and will protrude an extra 5mm into the cylinder.

I've tried to cross reference them but the only match for the Champion is the Denso T16PR-U11.

I got the plugs from Rockauto who up to now have been spot on with all their products so I don't know if the plugs I took out were wrong or if these are! Are these plugs going to damage my engine, has anyone got any clues?

Speed.

 

I think I might know what's going on here:

My '92 Lumina has a 3.1L with aluminum Gen II heads and uses NGK TR4's (physically the same plugs as Denso T16EPR-U). IIRC, the Gen II heads were only used on engines installed in FWD vehicles. The RWD vehicles continued to use cast iron heads.

The mix up might be due to an error on the part of manufacturers in differentiating between cast iron and aluminum head engines. Their application and cross reference are sort of whacked.

Boiled down, if your 3.1L has cast iron heads, I think you should be using either Champion RV15YC4, NGK UR4 or Denso T16PR-U (note the -11 suffix denotes a wide gap 0.044" plug).


Aluminum head plug on the left; cast iron plug on the right.

Coincidentally, the UR4 is the plug I use in my 5.0L V8 (by application).

 

Absolutely top bloke Paulo!

Thank's for that confirmation, I knew there was something not right with these plugs! I thought the threads looked like they extended too far into the cylinder, thank god I kept hold of the old one's and compared them before I tried to fit them. I'll get on to Rockauto and let them know before someone fit's them to their engines! Unfortunately I'm stuck now with 6 plugs I'm never going to use!

Thank's for the reply!

Speed

 

Got a reply from Rockauto last night, get this.

I contacted Denso regarding this fitment issue.* I went over all of the information provided in your email & was informed that their specification matches Pontiac's specs.* The technician informed me that they only have information on the North American market heads.* They did not have information related to European specification heads.* They believe European heads are different than the North American market heads, which would explain why the received plugs are a different length

So what gives here then? I've called the only importers of American cars here in England and they say that save for the various security systems which differ here in Europe all our cars are U.S. Spec.

Also my car was imported by a guy in the armed forces, he bought it in the USA so it too must be US spec also.

Speed.

 

Rock Auto is usually right on, but I'm calling them out on their explaination.

If they truly are using "Pontiac's specs", why does the Rock Auto catalog show the Champion No. 18 (RV15YC4) and the Denso 5022 (T16EPR-U) which is contradictory. So what they're saying is GM is confused?

I'd also call them out on the different heads for domestic (US) and export market explaination. Especially since your Firebird was built for the US market.

This confusion is not limited to just spark plugs and aluminum/iron heads. I've run into this problem with Fel-Pro intake manifold gaskets as well.

 

Yeah I'm pretty surprised at Rockauto for this too Paulo, I've been buying from them for over five years now and this is the only time I've ever had the wrong item(s) sent.
I'm not wanting money back or anything because of this (although a discount on my next order would have been nice) but I worry about what would have happened if I'd have put these 'wrong' longer plugs in!!
I would have thought that they would have at least removed these from the part list as they're clearly different from the one's for the 3.1 RWD.
As far as I'm aware, and I'm pretty sure on this from talking to the dealers here that all the Firebirds sold here in the 80's-90's were all U.S. spec and no "Euro" heads were ever made.

Thank's for the input Paulo it's much appreciated.

Speed.