97' blazer oil issue?
August 13, 2010 5:46 PM   Subscribe

My girlfriend's 97' chevy blazer sputters and looses power when under load (like going up a big hill), but once the oil is changed it runs fine. What is going on here???

When she first told me about this I was skeptical! Apparently it has happened the entire time she has had it (bought it used). This is how she knows its "ready for an oil change".

Before I changed the oil I checked the level, it was very low so I filled it back up, but it still had the same issue. A few days later I gave it an oil and filter change and sure enough, it runs fine!

Ive never seen anything like it, nor has my dad and uncle who have been working on cars their entire lives.. Any ideas?
posted by Esefa to Travel & Transportation (7 answers total)
The fuel injection system on that era of Blazer is very prone to problems. Instead of having an individual injector for each cylinder, inside the intake manifold is one central injector with tubes going to poppet valves at each cylinder. This system is called Central Port Fuel Injection and is different than the Multi-Port fuel Injection fitted to basically every other car.
Since a portion of the CPFI system is mechanical (poppet valves) it's really finicky about the fuel pressure. MPFI systems run at around 35-40psi, but the CPFI system needs over 50psi, otherwise it doesn't work correctly. It doesn't have any tolerance for low fuel pressure. Added to that, is that the system is also prone to leaking inside the intake manifold, which causes the car to run really rich and suffer from poor performance. Imagine just pouring straight gas into an engine while it's running.
This fuel injection system is the single leading cause of drivability and performance concerns when it comes to the Chevy Blazer. I'd bet it has something to do with this. Six out of ten Blazers that I've seen with rough running or loss of power or hard starting concerns turn out to be this injector. It's pretty notorious. Unfortunately, it's an expensive fix.
Of course, get it checked out by a professional. They'll probably do a fuel pressure test, a fuel pressure leak down test, and then check the fuel-trim related data in the engine control computer.
posted by Jon-o at 5:57 PM on August 13, 2010 [1 favorite]

I had a 95 blazer with this exact problem...There is a tech service bulletin out on the CPFI for blazers in that neighborhood of years...(not 100% sure 97 is included) and I got them to replace it at no charge at my dealership. I had to replace the fuel pump 6 times in as many years. POS truck in my opinion. Have a pro check the injector and the fuel pump..and be prepared for a few hundred dollars to fix it.
posted by AltReality at 6:05 PM on August 13, 2010

Thanks to both of you for the quick and detailed responses! However im struggling to connect this fuel injection issue to changing the oil. This problem literally only happens when the oil is at the "end of its range" (around 3 months or 3k miles), and goes away completely with fresh oil.
We havent had it looked at by the pros yet because we were expecting to sell it by now, but our plans have been changed a bit so we will have it a little while longer, grrrr
posted by Esefa at 6:17 PM on August 13, 2010

It might get worse as the oil gets older because when the CPFI leaks in the manifold and the engine runs rich for a prolonged period of time, the oil can become saturated with the excess fuel and then release fuel vapors back through the PCV system and exacerbate the already rich condition. This phenomenon can effect otherwise healthy cars that are subjected to a lifetime of short trips and cold starts.
Also the fuel breaks down the oil and reduces it's lubricating properties and thins out the viscosity. If the oil is too thin, it might effect the hydraulic lifters, causing the engine to run poorly. But that's a really extreme case.
posted by Jon-o at 6:33 PM on August 13, 2010 [2 favorites]

Does the oil smell strongly of gasoline when it's changed? Jon-o is correct about the leaky CPFI poppets. Blazers from that era also have fuel pressure regulators that are prone to failure (by leaking fuel into the manifold and causing identical rich conditions and fuel-saturated oil).

I would recommend taking a sample of the oil and having it analyzed professionally for traces of gasoline, coolant, combustion byproducts, etc. You can ship it off to a company like Blackstone Labs for a nominal fee. If it shows abnormally high levels of something, that'll point you in the right direction.

If you find that it's a fuel problem, a conversion kit to the upgraded MPFI system (which includes a new fuel pressure regulator) can be had though MGPartsDirect.com with part number 19210688 for about $250. It's actually a very simple process to do yourself. MeMail me for more info.

If you find there are even traces of coolant in the oil, these engines are also prone to leaking lower intake manifold gaskets (supposedly this is caused by GM's Dexcool breaking down the gaskets) which will allow coolant to leak outside of the engine, into combustion chambers, or directly into the crankcase. These are more difficult to change by yourself, but not impossible.

Good luck!

P.S. If you ever find that you need to replace the fuel pump, make sure either an ACDelco pump or a Delphi pump goes in. Any other brand and you'll be replacing it again in short order.
posted by Capa at 7:00 PM on August 13, 2010

I'm thinking jon-o is right on the fuel dilution thing. What happens is that when the engine is turned off (or even when it is running in some cases), the gasoline leaks out of the faulty injector or poppet, and down into the engine and dilutes the oil. Some of it burns and evaporates off again, but it does build up over time. So over the 3 months 3000 miles, combined with the engine burning/leaking oil, what was in the crankcase was probably as much gasoline as oil. Get it figured out asap, because (if this is the case), running the engine on such an out-of-spec lubricant is probably going to kill it sooner rather than later.

Other thought: if the oil was *very* low, and it always gets that way by the end of the cycle, what's left in there by the end is in such bad shape that it barely functions as oil at all. (Because some of what happens to oil that makes it go bad depends on the volume of oil in the car. The less oil there is, the more quickly is gets used up.)

The reason for either of these things is that all the moving parts in the engine are put together with the right gaps and tolerances so that when the right oil is in the right place, the pieces never even really touch. They just glide over each other on a bed of oil. If the oil is diluted, or so dirty and heat sheared as to not be doing its job, these pieces start to develop far more friction. Which causes more heat, which engines don't like. Which could cause the issues you report.
posted by gjc at 7:16 PM on August 13, 2010

Wow thanks to all, tons of great info, written in understandable terms!
This is not the only issue with the car, its just the biggest and until now was undiagnosed unlike the others.
So at this point it seems like fixing it would be fairly straight forward and maybe around $300 by the time its said and done (thanks capa).. but with all the other probs we may just keep fresh(er) oil in it until its traded in.. not sure, well have to figure out options..
Thanks again this is by far the most usable info ive got on this problem!
posted by Esefa at 7:52 PM on August 13, 2010

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