loud vacuum like noise when accelerating

Hi, im sorry if someone esle has this bit i could not find anyhting. I have a 1998 4X4 blazer. For a couple of months now (not all the time, it only does it sometime) when i accelarate it kinda seems (forgive my loss of words when it comes to describing car stuff) like it holds back a bit, doesnt respond and the engine (i think its the engine) makes like a loud vacuum (house vacumm) noise. Like theres a tornado going on in there... and its keeps doing it till i floor it and then it goes away. But when i come to a stop and start again the problem is back. The rpm's look like they're a bit high too. Does anyone have any idea what i just said??? And if u did understand my girl car talk, could u suggest what i should look into? Thanks for ur time..

RPM should hold at about 650 or so when idle. The loud roaring/vacuum noise you hear may be just a fan clutch issue. Does the noise go away when you hold engine RPM at about 2500? Does it make this noise at startup, roar a little bit and then stop, only to roar again at take-off when you pull away from the stop? Does it eventually go away after a few start/stops or does it keep doing it? And what are your local temps like? It's been 98 degrees in the shade here, swampy hot too. When I have been running the truck more than 15 minutes with the A/C going full tilt, she roars at take off every time. The engine is just hotter than usual in these conditions and to bleed off that heat, the clutch engages and spins the fan up. When she cools enough, the fan slows and it's all back to normal. This would be normal behavior, what you describe; if it is really hot where you live right now and you have your A/C running.

hmmmm? a lot of questions. Hahahahaha. It doesnt do it at start up, only when i accelerate after starting it. It doesnt do it for while the stops.Yesterday it did it from work to home and thats like 15 miles. I did/do have the AC going. It is very very hot here... 108 yesterday i believe. I just seems like my car is struggling. Poor thing.

It's most likely just normal operation of the fan clutch. When it's hot it engages the clutch and turns the fan faster. The extra load from the fan (not to mention the A/C compressor) will raise the idle a bit. When you accelerate and really get that fan turning you get quite a loud roaring noise. It goes away when you floor it because the higher rpms = higher fan speed = faster air flow through radiator = radiator cools + air flowing trough it doesn't get as hot = fan clutch thermostat cools down and disengages the clutch. There' will be a noticable loss of power because that fan takes a good bit of power to turn. Open the hood when the engine is hot and have somebody rev the engine to ~2000 rpm in park or neutral (with parking brake on for safety). You should be able to easily identify that it's the fan making all the noise.

Yeah, I agree with Wolfox and Tangent, it could just be the fan.

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makes sense....but i heard it could be the torque converter?? any relation there? Just something i heard. But thanks to all you who've replied. Eased my mind (and pocket).

Yeah, the TSB above that Blazee posted addresses that concern. (Busy/Cycling of TCC) Since this is an "Issue" you notice when at rest, or taking off, it's not directly related at all, just *seems* to be. In short, nothing to worry about at all. Running your A/C in 100+ ambient temps is going to make for some noise because when at rest, the truck does not have the air from you moving to force air through the cooling system. It makes up that lack of airflow by engaging your fan clutch. The clutch turns the fan at the same speed as your engine, making a TON of noise, but it's just doing what it's meant to do. When she's cooled off, or if you can maintain speeds above 40 MPH or so, she will not make so much noise because of the natural airflow from movement is doing the job of keeping your engine cool.

The hissing noise makes me think you have a bad vacuum leak somewhere.

Check all vacuum hoses going too and from the engine.

I have exactly the same problem. My question to pixie is this: Does your 4wd function properly? My doesn't and I have the same sound and issues. I just finished crawling under my car and located a burned in half vacuum hose. This hose is one of three that connects the vacuum actuator on the transfer case to componants in the engine compartment. I suspect this is my problem. Unfortunately I cannot locate the 2nd half of the vacuum hose to repair so it's going to the shop tomorrow. It's probably somewhere between the engine and the firewall and I just can't get an eye on it. If anyone has an opinion I would greatly apprciate it.

My gut level feeling is the 4wd is going in and out of gear due to t
he loss of vacuum. That is the sound I believe I'm hearing.

Although the fan clutch issue may be your problem, I would look at the vaccum hoses also.

I have a 98 and it roars every now and then and it is the fan, seems like you have nothing to worry about. Been really hot here lately so it does it more and more but truck is running a o.k. and is only going a little above the normal temp with the ac on in this brutal hot and muggy heat.