Pre-ignition starting

[cc98ranger]

My 4.0 liter in my 98 Ranger has started randomly having some pre-ignition when accelerating which it has never done before. The truck has 100,000 miles and this started this summer after having some work done to replace the fuel pump and filters. it doesn't happen all the time which is puzzling. There is no check engine light lit, no codes present, I checked that the EGR valve is working and it holds vacuum which all is ok there. Throttle body and intake are clean, have used different brands of fuel and today have decided to try 93 octane to see if the problem persists. Nothing with timing has been touched, fuel pressure is good, what am I missing? Thanks for any input.

 

[RonD]

You aren't missing anything, well maybe one thing.
4.0l OHV never had user adjustable spark timing(unless you reprogram computer).

The EGR system is there to lower NOx emissions which spike high when cylinder temps do.
When you accelerate or otherwise add a load to the engine the cylinders heat up, adding exhaust gases cause slower burning of air/fuel mix in cylinders which lowers the temp.
Side effect is less or no pinging which can be caused by higher cylinder temps, octane is a heat rating, the lower the octane the lower the self ignition temp, if cylinder temp gets too high the 87 octane can self ignite, pre-detonate, ping, or knock.

So you looked at that possible cause.
Did you put a vacuum on EGR valve with engine idling, engine should start to stumble as EGR valve opens?


Another one is a Lean air:fuel mix, lean mix will self ignite at a lower temp.
O2 sensor should "see" too much oxygen in exhaust(lean burn) and computer should then add more fuel to compensate for that, but that should happen is a second or two, unless engine is cold, O2 sensor don't work until they are above 600degF so can't take a few minutes to warm up.
Could be O2 sensor on one bank is "seeing" false Rich so computer is running that bank slightly lean, and when you accelerate you get pinging.

Might check Fuel Pressure.
New no longer means "it works"
New now means "we never tested it but do give you a warranty"

After engine is warmed up, unplug IAC valve, idle should drop down to 500rpm or engine may even stall, either is good, it means no vacuum leaks.
If idle stays above 650 then you could have a vacuum leak.

 

[cc98ranger]

Thanks for the reply, I will do some checking especially the unplugging IAC and see what happens. I did put vacuum to the EGR valve and it did hold vacuum but I did not try it with the truck running, I will try that as well. Your right about the fuel pump, maybe pressure is still not up to par. The O-2 sensors were all replaced last year. truck has never had a programmer put into it. Thanks for the input.

 

[cc98ranger]

Thought I would give an update on my problem. I checked everything and it was crazy because everything was checking out ok. This past summer I had the fuel pump and filters replaced and the tech said he was having problems with the fuel trim readings, but he got it sorted out, but then the detonation started, it would come and go. I had the intake cleaned, throttle body, etc, still the it would ping from time to time. I started using premium and that made no difference. Still, I had no check engine light, no codes, nothing, so I decided to change out the mass sensor since everything else checked out, including the egr valve and the other components of the system. Well I am happy to report problem solved and the truck runs great, now I can will try and go back to regular fuel once again. The truck has 101,000 miles and up to this fall has never had any issues with it. Given the sensor is 18 years old I am sure it was slowly failing but not to the point of throwing a code. I have slowly replaced many of the other sensors since their age brings into question their ability to work properly.

 

[RonD]

MAF issue for pinging?

Can't think that one thru to make sense.

MAF measures the weight of the air coming in
Computer uses that to calculate air/fuel mix
Mix is burned
O2 sensor reads oxygen level in exhaust

I can see O2 sensor causing Lean pre-ignition
But O2 should correct MAF sensor Lean, which is does and computer sets p0171/0174 codes.

I believe it is fixed, just don't know why it is fixed, lol.

 

[cc98ranger]

It doesn't make a lot of sense to me either, but it was the one thing that does have an effect with the fuel trim tables and since it was nearly 19 years old I figured it was getting weak but not to the point of generating a code yet. It was strange how the truck would run fine one moment then start pinging out of no where. As I said earlier, the tech that worked on the truck really began to have his doubts that the MAF was slowly dying but was reluctant to just throw parts at it. He would run it with live streaming and he could see the fuel trim do some really strange readings. Oh, well, as long as it is running good again, I will not complain about spending 85.00 to fix it myself. Thanks for the input you gave me since you gave me good info on what to check before throwing parts at it.

 

[msubully82]

I have experienced the same thing on a 1994 4.0L. Pinging and no codes or CEL, but running premium gas would make the pinging go away. I had already replaced the MAF sensor 2-3 years before, so I thought surely not again. Decided to just run premium gas, which worked for a few months. MAF finally started dying completely - stumbling, setting CEL, etc. after engine was warm. Unplug MAF and no more stumbling. Replaced MAF and stumbling went away, and it also solved the pinging problem with regular gas.

I have also had pinging caused by a dirty MAF.

 

[chewy012]

Did any of you check your engine temp. Sending unit?

I had a case of the pings, I read somewhere, but can't remember where, that when that gets gnarly it can tell the computer you're overheating and cut fuel, causing lean mix.

I changed out mine and it cured the pings. Also, mine was brown and pitted, new one was smooth and copper haha.

Anyway, food for thought

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